[CLEANUP] CharsetConverter code cleanup

* Correct phpDoc
* Use strict comparison where possible

Resolves: #62703
Releases: master
Change-Id: Iea29cdb9593a17529dee72cecce67c51f03da367
Reviewed-on: http://review.typo3.org/33799
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>