Fix filters.Mention matching other users without username - #5385

filters.Mention(user) compares user.username with the username of every text-mentioned user. When the given user has no username, that comparison is None == None. So the filter matches a text mention of any user without a username, even when the user ids differ:

alice = User(1, "Alice", False)  # no username
bob = User(2, "Bob", False)      # no username
msg = Message(1, now, Chat(1, "group"), text="Bob",
              entities=[MessageEntity(MessageEntity.TEXT_MENTION, 0, 3, user=bob)])
filters.Mention(alice).check_update(Update(1, message=msg))  # True, should be False

Telegram uses text mentions for users who don't have a username, so this is the common case for them. The existing tests didn't catch it because every text-mentioned user in them has a username.

With this change, usernames are only compared when the given user actually has one. Matching by id and by @username mention is unchanged.

Check-list for PRs

• Created new or adapted existing unit tests
• Checked the Stability Policy in case of deprecations or changes to documented behavior (bug fix only, no API change)