upb: clean up open vs closed enums in the Dart gencode

PiperOrigin-RevId: 619196103

upb/mini_descriptor/link.c

@@ -114,9 +114,7 @@ bool upb_MiniTable_Link(upb_MiniTable* m, const upb_MiniTable** sub_tables,
     if (upb_MiniTableField_CType(f) == kUpb_CType_Message) {
       const upb_MiniTable* sub = sub_tables[msg_count++];
       if (msg_count > sub_table_count) return false;
-      if (sub != NULL) {
-        if (!upb_MiniTable_SetSubMessage(m, f, sub)) return false;
-      }
+      if (sub && !upb_MiniTable_SetSubMessage(m, f, sub)) return false;
     }
   }
 
@@ -126,11 +124,9 @@ bool upb_MiniTable_Link(upb_MiniTable* m, const upb_MiniTable** sub_tables,
     if (upb_MiniTableField_IsClosedEnum(f)) {
       const upb_MiniTableEnum* sub = sub_enums[enum_count++];
       if (enum_count > sub_enum_count) return false;
-      if (sub != NULL) {
-        if (!upb_MiniTable_SetSubEnum(m, f, sub)) return false;
-      }
+      if (sub && !upb_MiniTable_SetSubEnum(m, f, sub)) return false;
     }
   }
 
-  return true;
+  return (msg_count == sub_table_count) && (enum_count == sub_enum_count);
 }