[BUGFIX] RTE Spellcheck: PHP warning in SpellCheckingController

PHP warning in raised in
SpellCheckingController::fixPersonalDictionaryCharacterSet.
If the file does not exist, there is no need to fix anything.

Releases: master, 6.2
Resolves: #62137
Change-Id: I9b15f5cb016ac9d309fb19ad6230a79b885b16f3
Reviewed-on: http://review.typo3.org/33239
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>