mypy: Correct SVN integration & satisfy strict-optional typing.

The commit_notice_destination return value was previously not
checked against None, which indicates no message should be sent.

zulip/integrations/svn/post-commit

@@ -66,6 +66,7 @@ message = "**{0}** committed revision r{1} to `{2}`.\n\n> {3}".format(
 
 destination = config.commit_notice_destination(path, rev)  # type: Optional[Dict[Text, Text]]
 
+if destination is not None:
     message_data = {
         "type": "stream",
         "to": destination["stream"],