[BUGFIX] EXT:indexed_search: Ensure that sword is a string

Resolves: #81782
Releases: master,8.7
Change-Id: I3bd19d8b796f7ec0d64961d45cb6937da2a06301
Reviewed-on: https://review.typo3.org/53394


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Sascha Rademacher <sascha.rademacher@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>