[BUGFIX] FAL: Prevent invalid NULL value on metadata creation

The cruser_id field is now guaranteed to be an integer.
This patch checks if a BE_USER is available and uses zero
otherwise.

Resolves: #57495
Related: #46020
Releases: 6.2
Change-Id: I8260604eb30ca32d2a5e079fa6f5b10a6d04714e
Reviewed-on: https://review.typo3.org/29028
Reviewed-by: Nicole Cordes
Reviewed-by: Jigal van Hemert
Reviewed-by: Frans Saris
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Markus Klein
Tested-by: Markus Klein