[BUGFIX] Implode array values in SaveToDatabaseFinisher

Array values being processed by SaveToDatabaseFinisher are now
imploded by a comma. This may happen when using a `MultiCheckbox` element.

Resolves: #82061
Releases: master, 8.7
Change-Id: Ic84af11ba00611aa89b043cd3b1c36dedb356c68
Reviewed-on: https://review.typo3.org/53664


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>