TST: GH30999 address all bare pytest.raises in pandas/tests/series and add EMPTY_STRING_PATTERN to tm
This PR is to address xref #30999 in pandas/tests/series

Three of the four errors raised within `pytest.raises` were for `NotImplementedError` with the empty string as the error message. Rather than individually making a regular expression for the empty string, I added one as a constant to `pandas/_testing/__init__.py` . There are two more similar cases in other modules so I wanted to put it somewhere that all the testing modules import. But if you don't like it I can revert it and instead use an individual empty string pattern in each case.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks