[BUGFIX] Missing TSFE renderCharset initialization

This patch adds the initialization of TSFE->renderCharset which is used
in some functions.

Resolves: #59749
Change-Id: I17f58fc8457e43af340d5acd1069fcba7cfb7d3a
Reviewed-on: https://review.typo3.org/30971
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee