Handle None for parse_inbound (#517)

parse_inbound takes the `message` from `self._parse_inbound`, which can
in some cases be None, causing issues when we try to access `.url` and
`.text` off it.

Author: GitOnUp

src/steamship/agents/mixins/transports/transport.py

@@ -56,6 +56,8 @@ def _send(self, blocks: List[Block], metadata: Metadata):
 
     def parse_inbound(self, payload: dict, context: Optional[dict] = None) -> Optional[Block]:
         message = self._parse_inbound(payload, context)
+        if not message:
+            return None
 
         if message.url and not message.text:
             context = AgentContext()