Validate input parameters for the sendToDevice API. (#8975)

This makes the "messages" key in the content required. This is currently
optional in the spec, but that seems to be an error.

changelog.d/8975.bugfix

@@ -0,0 +1 @@
+Add validation to the `sendToDevice` API to raise a missing parameters error instead of a 500 error.

synapse/rest/client/v2_alpha/sendtodevice.py

@@ -17,7 +17,7 @@
 from typing import Tuple
 
 from synapse.http import servlet
-from synapse.http.servlet import parse_json_object_from_request
+from synapse.http.servlet import assert_params_in_dict, parse_json_object_from_request
 from synapse.logging.opentracing import set_tag, trace
 from synapse.rest.client.transactions import HttpTransactionCache
 
@@ -54,6 +54,7 @@ async def _put(self, request, message_type, txn_id):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         content = parse_json_object_from_request(request)
+        assert_params_in_dict(content, ("messages",))
 
         sender_user_id = requester.user.to_string()