BUG: read_excel with openpyxl produces trailing rows of nan
- [x] closes #39181
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

This is on top of #39486
why is this not an upstream bug? (not averse to fixing in pandas at least tactically). @jreback [openpyxl docs](https://openpyxl.readthedocs.io/en/stable/optimized.html):

> Read-only mode relies on applications and libraries that created the file providing correct information about the worksheets, specifically the used part of it, known as the dimensions. Some applications set this incorrectly. You can check the apparent dimensions of a worksheet using ws.calculate_dimension(). If this returns a range that you know is incorrect, say A1:A1 then simply resetting the max_row and max_column attributes should allow you to work with the file:

We could fix this issue by calling `sheet.calculate_dimension(force=True)` but this involves a 50% performance hit (measured on a frame with 10000 rows and 10 columns). The method used here involves a 2% performance hit. Ref: https://github.com/pandas-dev/pandas/issues/39001#issuecomment-770277269 > @jreback [openpyxl docs](https://openpyxl.readthedocs.io/en/stable/optimized.html):
> 
> > Read-only mode relies on applications and libraries that created the file providing correct information about the worksheets, specifically the used part of it, known as the dimensions. Some applications set this incorrectly. You can check the apparent dimensions of a worksheet using ws.calculate_dimension(). If this returns a range that you know is incorrect, say A1:A1 then simply resetting the max_row and max_column attributes should allow you to work with the file:
> 
> We could fix this issue by calling `sheet.calculate_dimension(force=True)` but this involves a 50% performance hit (measured on a frame with 10000 rows and 10 columns). The method used here involves a 2% performance hit. Ref: [#39001 (comment)](https://github.com/pandas-dev/pandas/issues/39001#issuecomment-770277269)

ok! yeah as long as its robust ok with this lgtm. @simonjayhawkins prob worth doing for 1.2.2 sure ```
==================================== ERRORS ====================================
__________ ERROR collecting scripts/tests/test_validate_docstrings.py __________
ImportError while importing test module '/home/runner/work/pandas/pandas/scripts/tests/test_validate_docstrings.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/share/miniconda/envs/pandas-dev/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
scripts/tests/test_validate_docstrings.py:5: in <module>
    import validate_docstrings
scripts/validate_docstrings.py:50: in <module>
    from numpydoc.validate import validate, Docstring  # isort:skip
E   ImportError: cannot import name 'Docstring' from 'numpydoc.validate' (/usr/share/miniconda/envs/pandas-dev/lib/python3.8/site-packages/numpydoc/validate.py)
=========================== short test summary info ============================
ERROR scripts/tests/test_validate_docstrings.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.54s ===============================
```

hmm is this a suprious issue on the ci / checks? numpydoc failure is also happening on master, so unrelated  @meeseeksdev backport 1.2.x