ValueError when grouping with max/min as aggregation functions (pandas-1.0.1)
#### Code Sample

```python
import pandas as pd # 1.0.1
import numpy as np # 1.18.1

# Simple test case that fails
df_simple = pd.DataFrame({'key': [1,1,2,2,3,3], 'data' : [10,20,30,40,50,60],
                          'good_string' : ['cat','dog','cat','dog','fish','pig'],
                          'bad_string' : ['cat',np.nan,np.nan, np.nan, np.nan, np.nan]})

df_simple_max = df_simple.groupby(['key'], as_index=False, sort=False).max()

>>>line 17, in <module>
    df_simple_max = df_simple.groupby(['key'], as_index=False, sort=False).max()

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/groupby/groupby.py", line 1378, in f
    return self._cython_agg_general(alias, alt=npfunc, **kwargs)

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 1004, in _cython_agg_general
    how, alt=alt, numeric_only=numeric_only, min_count=min_count

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 1099, in _cython_agg_blocks
    agg_block: Block = block.make_block(result)

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/internals/blocks.py", line 273, in make_block
    return make_block(values, placement=placement, ndim=self.ndim)

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/internals/blocks.py", line 3041, in make_block
    return klass(values, ndim=ndim, placement=placement)

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/internals/blocks.py", line 2589, in __init__
    super().__init__(values, ndim=ndim, placement=placement)

  File "/usr/local/anaconda3/lib/python3.7/site-packages/pandas/core/internals/blocks.py", line 125, in __init__
    f"Wrong number of items passed {len(self.values)}, "

ValueError: Wrong number of items passed 1, placement implies 2
-------------

# Add one more legitimate string value to the 'bad_string' column and it works
df_simple = pd.DataFrame({'key': [1,1,2,2,3,3], 'data' : [10,20,30,40,50,60],
                          'good_string' : ['cat','dog','cat','dog','fish','pig'],
                          'bad_string' : ['cat','dog',np.nan, np.nan, np.nan, np.nan]})

df_simple_max = df_simple.groupby(['key'], as_index=False, sort=False).max()

```
#### Problem description

Unless I've misunderstood something fundamental about the max, and min, aggregate functions, I don't think they should error out when a Series in the DataFrame is of type object and contains all but one `NaN` value. Notice from the example above that just adding one more non-`NaN` value to the offending Series gets around the `ValueError`. 

#### Expected Output

No `ValueError`; groupby object returned. 

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

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.2.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.6.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200210
Cython           : 0.29.15
pytest           : 5.3.5
hypothesis       : 5.4.1
sphinx           : 2.4.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.43.1
```
