audit logs owner instead of executant for file uploads

## Describe the bug

The FileUploadEvent has an owner property which is used is by the audit log to extract a uid. AFAICT the intention is to log who wrote a file. But in this case the owner is logged. The event also carries an Executant property, but it is ignored by the audit log.

## Steps to reproduce

Steps to reproduce the behavior:

1. upload a file into a received share
2. check the audit log

## Expected behavior
The executant should be logged.

## Actual behavior
The owner is logged.

## Course of action
The Owner property should be completely dropped, as a space may not even have an owner, only one or more managers.