Allow passing custom sentinel into MultiIndex.format
Commit b0468aa45f3 seems to have unintentionally removed functionality
to pass custom sentinel values for sparsifying in MultiIndex.format().
Previously, any "Truthy" value other than 1 and True was used as the
sentinel, which is useful particularly with adjoin=False. The current
logic does not make sense: it checks sparsify for "Falsey" values under
"if sparsify:" condition. Under this if sparsify cannot be Falsey.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Hello @eltoder! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-06-17 18:59:55 UTC @jreback I agree. Ok to keep this change though? The current code doesn't make sense and changed the API @jreback How about this? This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale, if you want to continue working, please ping.