helper/schema: if you can't find the list, just fall through to return

nil

helper/schema/schema.go

@@ -224,6 +224,8 @@ func (m schemaMap) diffList(
 		if !schema.Computed {
 			return nil
 		}
+
+		return nil
 	}
 
 	// We have to use reflection to build the []interface{} list