[BUGFIX] TS: Allow "0" as strPad.padWith

Replaces the test with empty() by isset() and
strlen() > 0. So other strings which are
handled by PHP as empty can be used for padding.

Resolves: #51650
Releases: 6.2, 6.1
Change-Id: Iee62f19f97b958ab2d02c6ca73052810a3c91117
Reviewed-on: https://review.typo3.org/25669
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind