[BUGFIX] Avoid to get a file object with the uid zero

At some points where a file object is retrieved, a check for
the interpretation as integer is done for the method
argument only. If the argument is 0 a exception will be
thrown from the ResourceFactory.
A file object should only be fetched if the uid is an integer
greater than zero.

Resolves: #55530
Releases: 6.2
Change-Id: I9399d58bac4a48344769ac00207b64e25eea630e
Reviewed-on: https://review.typo3.org/27304
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein