[BUGFIX] Avoid strict type exception in Import

$tokenizedContent could be null and would already cause an undefined
array key warning. When passed to the strlen method, this even causes
an exception.

This patch adds a null coalesce operator with an empty string as default
and replaces the odd strlen bool expression with a strict comparison to
an empty string.

Resolves: #102461
Releases: main, 12.4
Change-Id: I88464956395240990a51bdb334502cd58da347bf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81895


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>