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

@@ -66,10 +66,11 @@ message = "**{0}** committed revision r{1} to `{2}`.\n\n> {3}".format(
 
 destination = config.commit_notice_destination(path, rev)  # type: Optional[Dict[Text, Text]]
 
-message_data = {
-    "type": "stream",
-    "to": destination["stream"],
-    "subject": destination["subject"],
-    "content": message,
-}  # type: Dict[str, Any]
-client.send_message(message_data)
+if destination is not None:
+    message_data = {
+        "type": "stream",
+        "to": destination["stream"],
+        "subject": destination["subject"],
+        "content": message,
+    }  # type: Dict[str, Any]
+    client.send_message(message_data)