Form submits without a file cause a sys_file-reference with an empty sys_file relation. Currently this can not be prevented. But the reference can be decoupled from the event.
@see: https://forge.typo3.org/issues/85216#note-16

[WIP][FEATURE] Add a form to submit new events in the frontend. A new event is disabled by default and has to be enabled (=reviewed) by a team member. Once a frontend user has an event that has been approved, his future events are approved by default.

thx to Benni Mack