[CLEANUP] Code cleanup for LanguageService

* Avoid unnecessary fully qualified class names
* Add return NULL for consistency if a method only returns a value
  in some cases

Releases: 6.2
Resolves: #59006
Change-Id: I8c005a3e20fc49d65123e415025bef1c41ad7854
Reviewed-on: https://review.typo3.org/30260
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn