[BUGFIX] Improve handling of unmatched expressions

Currently the markers corresponding to unmatched expressions are
left unreplaced in the template code (and cleaned up later). To
be able to test for absent variable, we need some clear value
instead. This changes introduces the use of 'NULL'.

Resolves: #60472
Releases: 1.7
Change-Id: I76fff296460d6a53942596c14856396c2e19a406
Reviewed-on: https://review.typo3.org/31726
Reviewed-by: Francois Suter <francois@typo3.org>
Tested-by: Francois Suter <francois@typo3.org>