send_message_backend: Set the realm value passed down to check_message.

This leftover variable, as a result of older changes, was just always
set to None. That was fine, because when realm=None reaches
check_message further down the codepath, it just infers from
sender.realm. We want to stop passing None like that though, so let's
just set this to user_profile.realm.

zerver/views/message_send.py

@@ -237,7 +237,7 @@ def send_message_backend(
     if forged and not can_forge_sender:
         raise JsonableError(_("User not authorized for this query"))
 
-    realm = None
+    realm = user_profile.realm
     if realm_str and realm_str != user_profile.realm.string_id:
         # The realm_str parameter does nothing, because it has to match
         # the user's realm - but we keep it around for backward compatibility.