Fix parse_rfc3339 to handle None from timezone regex

Add defensive check to prevent AttributeError when _re_timezone.search()
returns None. Also handle space in timezone position gracefully by treating
it like 'Z' (UTC). Includes test case and clear error message.

Fixes issue where 'NoneType' object has no attribute 'groups' error could occur.

Co-authored-by: brendandburns <5751682+brendandburns@users.noreply.github.com>