ParseResultBytes.unsplit() raises AttributeError for a reference without an authority #163

>>> from rfc3986.parseresult import ParseResult, ParseResultBytes
>>> ParseResult.from_string("/a").unsplit()
'/a'
>>> ParseResultBytes.from_string(b"/a").unsplit()
AttributeError: 'NoneType' object has no attribute 'encode'

Any reference without an authority (/a, a, mailto:x, ?q) crashes the bytes variant, while the str variant round-trips. Reproduced on 2.0.0 (main 3bd3832).

The traceback: unsplit calls copy_with(lazy_normalize=False), which calls _generate_authority, which reads the authority property, and that property does self.reference.authority.encode(self.encoding) (parseresult.py:409) without checking for None. The other bytes properties (scheme, path, …) are guarded the same way authority should be.

Happy to send a PR with a test if you would take one.