[BUGFIX] Don't ignore file imports in fallback storage

An export could contain sys_file records that are "stored" in the
fallback storage (ID 0). They must not be ignored.

Resolves: #58841
Releases: 6.3, 6.2
Change-Id: I39855810621310831ab1353c5e56f4529a7ac3aa
Reviewed-on: https://review.typo3.org/30979
Reviewed-by: Markus Klein
Tested-by: Markus Klein