actions: Remove 'topic_name is None' check from can_edit_content_or_topic.

We do not need the 'topic_name is None' check in this function as this is
called only when atleast one of the content and topic_name is not None,
and this condition cannot be true as there is 'content is not None'
check just before it.

Thus, 'if topic_name is None' condition being true means that both content
and topic_name are None which is not possible as this function itself will
not be called in such case. An assert statement is added to check that
topic_name is not None to make sure that it is handled when the function
is called in some other way later.

zerver/lib/actions.py

@@ -2810,9 +2810,7 @@ def can_edit_content_or_topic(
     if content is not None:
         return False
 
-    # If no topic change is requested, we're done.
-    if topic_name is None:  # nocoverage
-        return True
+    assert topic_name is not None
 
     # The following cases are the various reasons a user might be
     # allowed to edit topics.