Attempts to fetch(None) get None.

It sometimes happens in the wild that someone submits "inReplyTo: null" or similar.

kepi/sombrero_sendpub/fetch.py

@@ -65,6 +65,9 @@ def fetch(address,
     On all other errors, which are logged, returns None.
     """
 
+    if address is None:
+        return None
+
     wanted = _parse_address(address)
 
     wanted['type'] = expected_type