### What changes were proposed in this pull request?
This PR enables plot tests with plotly
```bash
./python/run-tests --python-executables=python3 --modules=pyspark-pandas
```
**Before**:
```
Traceback (most recent call last):
File "/.../miniconda3/envs/python3.8/lib/python3.8/runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/.../miniconda3/envs/python3.8/lib/python3.8/runpy.py", line 87, in _run_code
exec(code, run_globals)
File "/.../pyspark/pandas/tests/plot/test_frame_plot_plotly.py", line 42, in <module>
plotly_requirement_message + " Or pandas<1.0; pandas<1.0 does not support latest plotly "
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
```
**After**:
```
...
Starting test(python3): pyspark.pandas.tests.plot.test_series_plot_plotly
...
Finished test(python3): pyspark.pandas.tests.plot.test_series_plot_plotly (23s)
...
Tests passed in 1296 seconds
```
### Why are the changes needed?
For test coverage.
### Does this PR introduce _any_ user-facing change?
No, test-only.
### How was this patch tested?
By running the tests.
Closes#32649 from HyukjinKwon/SPARK-35497.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
4a6d844184