BUG: `DataFrame.mean` fails but `Series.mean` works incase of pandas Nullable integer dtype
- [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
>>> import pandas as pd
>>> df = pd.DataFrame({"a":[1,2,3, 4], "b":pd.Series([1, 2, 4, None], dtype=pd.Int64Dtype())})
>>> df
   a     b
0  1     1
1  2     2
2  3     4
3  4  <NA>
>>> df.mean()
a    2.500000
b    2.333333
dtype: float64
>>> df.mean(skipna=False)
a     2.5
b    <NA>
dtype: object
>>> df.iloc[3]
a       4
b    <NA>
Name: 3, dtype: Int64
>>> df.iloc[3].mean(skipna=True)
4.0
>>> df.iloc[3].mean(skipna=False)
<NA>
>>> df.mean(axis=1, skipna=False)
Traceback (most recent call last):
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 1415, in _ensure_numeric
    x = x.astype(np.complex128)
TypeError: must be real number, not NAType

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/frame.py", line 8649, in _reduce
    result = func(values)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/frame.py", line 8570, in func
    return op(values, axis=axis, skipna=skipna, **kwds)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 71, in _f
    return f(*args, **kwargs)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 129, in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 563, in nanmean
    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 1418, in _ensure_numeric
    x = x.astype(np.float64)
TypeError: float() argument must be a string or a number, not 'NAType'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 1415, in _ensure_numeric
    x = x.astype(np.complex128)
TypeError: must be real number, not NAType

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/generic.py", line 11466, in stat_func
    func, name=name, axis=axis, skipna=skipna, numeric_only=numeric_only
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/frame.py", line 8660, in _reduce
    result = func(values)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/frame.py", line 8570, in func
    return op(values, axis=axis, skipna=skipna, **kwds)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 71, in _f
    return f(*args, **kwargs)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 129, in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 563, in nanmean
    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))
  File "/Users/pgali/PycharmProjects/del/venv1/lib/python3.7/site-packages/pandas/core/nanops.py", line 1418, in _ensure_numeric
    x = x.astype(np.float64)
TypeError: float() argument must be a string or a number, not 'NAType'


```

#### Problem description

When there is a row-wise mean calculation of a nullable interger dtype the operation fails. But the same operation works correctly on a series by doing an iloc on the row.

#### Expected Output

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8
pandas           : 1.1.2
numpy            : 1.17.3
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 50.3.0
Cython           : None
pytest           : None
hypothesis       : 5.29.0
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 1.0.1
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None


</details>
