[BUGFIX] Fix syntax and logic errors in BE Fluid templates

Fixes an invalid syntax occurrence in the "PageZero.html"
template from EXT:tstemplate. And fixes a logical error
plus incorrect XML tag nesting in the partial template
"DocHeader.html" from EXT:backend.

Change-Id: Ib38fbb2d80cd8e24a2400ec7d2dbd12dee5605ec
Resolves: #88422
Releases: master, 9.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60802


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jonas Eberle <flightvision@googlemail.com>
Tested-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Jonas Eberle <flightvision@googlemail.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>