[BUGFIX] Page-TSconfig ###PAGE_TSCONFIG_STR### not evaluated correctly

Page-TSconfig ###PAGE_TSCONFIG_STR### not evaluated
correctly in the backend.

Change-Id: Ieda08a1df71117b5ac90f2c68260f5171570ff63
Resolves: #16392
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/20901
Reviewed-by: Markus Klein
Tested-by: Markus Klein