Fix webhooks: they should return response.empty (no error) on github_post if it's not a valid PR

webhooks/requirements.txt

@@ -0,0 +1,9 @@
+jinja2
+sanic==21.12.2
+GitPython
+pyyaml
+toml
+websockets==10.0
+babel
+langcodes
+language_data

webhooks/webhook.py

@@ -73,6 +73,8 @@ async def github_post(request: Request) -> HTTPResponse:
 
         if valid_pr_comment:
             return on_pr_comment(request, pr_infos)
+        else:
+            return response.empty()
 
     return response.json({"error": f"Unknown event '{event}'"}, 422)