Increase Test Coverage of CallbackQueryHandler (python-telegram-bot#2520)

* Test: let's see

* Test: let's see, now in the correct place

* Fix: Explicitly return None in else clause

also documented this behaviour clearly in the docstring

* add link in doc

Co-authored-by: Bibo-Joshi <hinrich.mahler@freenet.de>

* Fix: remove unnecessary else statement

Co-authored-by: Bibo-Joshi <hinrich.mahler@freenet.de>

Author: Poolitzer

telegram/ext/callbackqueryhandler.py

@@ -81,7 +81,8 @@ class CallbackQueryHandler(Handler[Update]):
             DEPRECATED: Please switch to context based callbacks.
         pattern (:obj:`str` | `Pattern`, optional): Regex pattern. If not :obj:`None`, ``re.match``
             is used on :attr:`telegram.CallbackQuery.data` to determine if an update should be
-            handled by this handler.
+            handled by this handler. If :attr:`telegram.CallbackQuery.data` is not present, the
+            :class:`telegram.CallbackQuery` update will not be handled.
         pass_groups (:obj:`bool`, optional): If the callback should be passed the result of
             ``re.match(pattern, data).groups()`` as a keyword argument called ``groups``.
             Default is :obj:`False`

tests/test_callbackqueryhandler.py

@@ -135,6 +135,10 @@ def test_with_pattern(self, callback_query):
         callback_query.callback_query.data = 'nothing here'
         assert not handler.check_update(callback_query)
 
+        callback_query.callback_query.data = False
+        callback_query.callback_query.game_short_name = "this is a short game name"
+        assert not handler.check_update(callback_query)
+
     def test_with_passing_group_dict(self, dp, callback_query):
         handler = CallbackQueryHandler(
             self.callback_group, pattern='(?P<begin>.*)est(?P<end>.*)', pass_groups=True