[BUGFIX] Prevent exception when using link wizard in flexform input

The itemValue of a field might be null, which is not accepted
by getLinkExplanation(). Add a check to ensure we always pass a string.

Resolves: #79749
Releases: master
Change-Id: Ic1b80faf1e2cc8c4e4119238225c6480a66c4cdc
Reviewed-on: https://review.typo3.org/51628


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Dominique Kreemers <dominique.kreemers@icloud.com>
Tested-by: Dominique Kreemers <dominique.kreemers@icloud.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>