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.