Protect vs returning a None grpc message

Signed-off-by: Juanjo Alvarez <[email protected]>

ddtrace/contrib/grpc/client_interceptor.py

@@ -173,7 +173,7 @@ def __next__(self):
                 "grpc.response_message",
                 (n,),
             )
-            if result and "response" in result:
+            if result and result.get("response"):
                 n = result["response"].value
         return n