ENH: DataFrame.to_parquet() returns bytes if path_or_buf not provided
- [x] closes #37105 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Not sure if this is an API breaking change. Prior to this patch `path` was a required positional argument but it becomes optional here. It is now consistent with, for example, the csv writer.
lgtm. can you merge master and ping on green just to make sure.

@jorisvandenbossche if any comments. Thanks @arw2019 !