Do not attempt to produce "r_engine_stats" on the temporary (=work) tables.
These tables may be
- re-created during the query execution
- freed during the query execution (This is done e.g. in JOIN::cleanup(),
  before we produce ANALYZE FORMAT=JSON output).

- (Also, make save_explain_data() functions not set handler_for_stats
  to point to handler objects that do not have handler->handler_stats set.
  If the storage engine is not collecting handler_stats, it will not have
  them when we're producing ANALYZE FORMAT=JSON output, either).