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

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

---

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

```python
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()``

<details>

INSTALLED VERSIONS
------------------
commit           : 7d32926db8f7541c356066dcadabf854487738de
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

</details>