broker: fix RawMessage marshal

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

broker/broker.go

@@ -42,7 +42,7 @@ type RawMessage []byte
 // MarshalJSON returns m as the JSON encoding of m.
 func (m *RawMessage) MarshalJSON() ([]byte, error) {
 	if m == nil {
-		return nil, nil
+		return []byte("null"), nil
 	}
 	return *m, nil
 }