This commit asserts that parse_user_agent never returns None. The
RegEx will match any string, so that `match` is never None. This
brings test coverage of lib/user_agent.py to 100%. Changes were also
made in test/test_decorators.py and views/compatibility.py to reflect
that parse_user_agent cannot return None.
Improves: #7089.
Fixes: #8779.