[FEATURE] Integrate possibility to determine NULL values in TypoScript

Currently there's no possibility to handle NULL values in
TypoScript. This is related to the feature "Add possibility
to store NULL values (#41773)".

Thus, two new elements are integrated:
* (stdWrap).ifNull -> string/stdWrap
* (stdWrap).if.isNull -> stdWrap

Change-Id: I77a29341c66edaf3bef14de33f65c702fbbb9fbd
Resolves: #43133
Releases: 6.0
Reviewed-on: http://review.typo3.org/16637
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader