BUG: drop_duplicates() doesn't drop zero-column rows

[lealanko-rt]

• I have checked that this issue has not already been reported.

• 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.

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Code Sample, a copy-pastable example

import pandas as pd
df_01 = pd.DataFrame([[]])
df_02 = pd.DataFrame([[],[]])
assert df_02.drop_duplicates().equals(df_01)
assert df_02.duplicated().equals(pd.Series([False,True]))

Problem description

Duplicates checking fails on various zero-column dataframes. They check for self.empty for a fast-path bypass, when they really should be checking e.g. self.index.empty.

Expected Output

The above assertions should succeed.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 7d32926
python : 3.7.3.final.0
python-bits : 64
OS : Linux
OS-release : 4.4.0-17763-Microsoft
Version : #864-Microsoft Thu Nov 07 15:22:00 PST 2019
machine : x86_64
processor :
byteorder : little
LC_ALL : C
LANG : en_US.UTF-8
LOCALE : None.None

pandas : 1.2.2
numpy : 1.20.1
pytz : 2021.1
dateutil : 2.8.1
pip : 18.1
setuptools : 40.8.0
Cython : None
pytest : None
hypothesis : None
sphinx : 3.4.3
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.4.2
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 5.8.0
pandas_datareader: None
bs4 : 4.7.1
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.0.2
numexpr : 2.6.9
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.1.0
sqlalchemy : None
tables : 3.4.4
tabulate : 0.8.2
xarray : None
xlrd : None
xlwt : None
numba : None

[simonjayhawkins]

Thanks @lealanko-rt for the report. This is a duplicate of #12869. There is also some discussion there on what the result should be. closing this to help keep discussion in one place.