[BUGFIX] Fix saving non public export presets

Fix sql error "Column 'public' cannot be null" on saving
with unchecked box public.

Resolves: #57405
Releases: 6.2
Change-Id: I1582728cc56781fdc12409956008fc82a6cd0d03
Reviewed-on: https://review.typo3.org/30112
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Markus Klein
Tested-by: Markus Klein