[BUGFIX] Wrong usage of intExplode in getTreeList

getTreeList calls intExplode which converts empty
arrays to 0. This patch removes empty arrays
within intExplode.

Resolves: #55384
Releases: 6.2
Change-Id: Id4ca1a15edf2cc2617d85bda765461c4cb1f105c
Reviewed-on: https://review.typo3.org/27089
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack