[BUGFIX] Init variable in GeneralUtility::createVersionNumberedFilename

The variable '$doNothing' was not initialized, leading to PHP notices.

Resolves: #60116
Releases: 6.3, 6.2
Change-Id: I38473b73d4b1ebeb4b1dcbea5ba24d005c520487
Reviewed-on: https://review.typo3.org/31346
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring