just use identity for the default implementations of api-serialize

src/metabase/models/interface.clj

@@ -5,10 +5,8 @@
     "Called on all objects being written out by the API. Default implementations return THIS as-is, but models can provide
      custom methods to strip sensitive data, from non-admins, etc."))
 
-(extend-protocol IModelInstanceApiSerialize
-  Object
-  (api-serialize [this]
-    this)
-  nil
-  (api-serialize [_]
-    nil))
+(extend Object
+  IModelInstanceApiSerialize {:api-serialize identity})
+
+(extend nil
+  IModelInstanceApiSerialize {:api-serialize identity})