[BUGFIX] Fix undefined array key in FrontendGenerationPageIndexingTrigger

This is an edge-case, where `config.index_enable = 1` is set, but the
static TypoScript from indexed_search is not included and thus
`config.index_externals` is undefined.

Resolves: #100888
Releases: main, 12.4, 11.5
Change-Id: I0b9fa04970812ca82edc4d223cd57e59e3a2ed83
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79070


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>