[BUGFIX] Indexed Search: Only write internal log if `debugMode` is enabled

The `Indexer` class from Indexed Search has an `internal_log` property
that's only accessed when `debugMode` is enabled.

However, the wrapper method `log_setTSlogMessage()` always writes into
the log, which doesn't make much sense. Said method now checks for
enabled debugging before writing into the log property.

Resolves: #102909
Releases: main, 12.4
Change-Id: I3d2df9ce49677e995bc86e87d74d80a9016900c4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82615


Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>