BUG: Series has no attribute "reshape" after adding a new category in df
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
# Your code here
import pandas as pd
from pandas.api.types import CategoricalDtype

# create dataframe (note: every single column is a category)
df =   pd.DataFrame(
               {
                    "a": pd.Series([np.nan, 2.0, 3.0, 1.0]).astype("category"),
                    "b": pd.Series(["A", "A", "B", "C"]).astype("category"),
                    "c": pd.Series(["D", "E", "E", np.nan]).astype("category"),
                }
      )

# get all categories from column "a"
cats =df["a"].cat.categories.tolist()
# append new category 
cats.append(0.0)
df["a"] = df["a"].astype(CategoricalDtype(categories=cats, ordered=False))
# fillna with that new category
df["a"].fillna(0, inplace=True)
# run df.isnull()
df.isnull().sum()
```

#### Problem description

We are trying to add the new fillna as a new category in the dataframe, but it fails when we are trying to use `df.isnull()`  In this case we are pretty much blocked from using df.isull().sum() functionality. Running the above snippet will get us the attribute error
`AttributeError: 'Series' object has no attribute 'reshape'`

Full stack trace is shown below.

```
AttributeError                            Traceback (most recent call last)
<ipython-input-29-cc81fd27034f> in <module>
----> 1 df.isnull()

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/frame.py in isnull(self)
   4865     @doc(NDFrame.isna, klass=_shared_doc_kwargs["klass"])
   4866     def isnull(self) -> "DataFrame":
-> 4867         return self.isna()
   4868 
   4869     @doc(NDFrame.notna, klass=_shared_doc_kwargs["klass"])

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/frame.py in isna(self)
   4860     @doc(NDFrame.isna, klass=_shared_doc_kwargs["klass"])
   4861     def isna(self) -> "DataFrame":
-> 4862         result = self._constructor(self._data.isna(func=isna))
   4863         return result.__finalize__(self, method="isna")
   4864 

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/managers.py in isna(self, func)
    500 
    501     def isna(self, func) -> "BlockManager":
--> 502         return self.apply("apply", func=func)
    503 
    504     def where(

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/managers.py in apply(self, f, align_keys, **kwargs)
    394                 applied = b.apply(f, **kwargs)
    395             else:
--> 396                 applied = getattr(b, f)(**kwargs)
    397             result_blocks = _extend_blocks(applied, result_blocks)
    398 

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/blocks.py in apply(self, func, **kwargs)
    346             result = func(self.values, **kwargs)
    347 
--> 348         return self._split_op_result(result)
    349 
    350     def _split_op_result(self, result) -> List["Block"]:

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/blocks.py in _split_op_result(self, result)
    361 
    362         if not isinstance(result, Block):
--> 363             result = self.make_block(result)
    364 
    365         return [result]

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/blocks.py in make_block(self, values, placement)
    250             placement = self.mgr_locs
    251         if self.is_extension:
--> 252             values = _block_shape(values, ndim=self.ndim)
    253 
    254         return make_block(values, placement=placement, ndim=self.ndim)

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/internals/blocks.py in _block_shape(values, ndim)
   2745             # block.shape is incorrect for "2D" ExtensionArrays
   2746             # We can't, and don't need to, reshape.
-> 2747             values = values.reshape(tuple((1,) + shape))  # type: ignore
   2748     return values
   2749 

~/.pyenv/versions/3.7.0/envs/fair_ml/lib/python3.7/site-packages/pandas/core/generic.py in __getattr__(self, name)
   5128             if self._info_axis._can_hold_identifiers_and_holds_name(name):
   5129                 return self[name]
-> 5130             return object.__getattribute__(self, name)
   5131 
   5132     def __setattr__(self, name: str, value) -> None:

AttributeError: 'Series' object has no attribute 'reshape'

```
#### Expected Output
a 0
b 0
c 1

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.0.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Sun Jul  5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 47.3.1
Cython           : None
pytest           : 5.4.3
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.13
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
