Add ENTITY_HAS_NO_PROPERTIES message_code

proto/caosdb/entity/v1alpha1/main.proto

@@ -36,6 +36,8 @@ enum MessageCode {
   MESSAGE_CODE_UNKNOWN = 1;
   // The requested entity (retrieve, delete, or update) does not exist.
   MESSAGE_CODE_ENTITY_DOES_NOT_EXIST = 2;
+  // The entity has no properties
+  MESSAGE_CODE_ENTITY_HAS_NO_PROPERTIES = 3;
 }
 
 // Messages are used by server's or client's to transport *transient*