[BUGFIX] Serialize expression in switch viewhelper

The f:switch ViewHelper breaks with Fluid error "No value found for key
"TYPO3\CMS\Fluid\ViewHelpers\SwitchViewHelper->switchExpression", thus
the key cannot be removed." for a NULL expression. This patch serializes
the expression before storing it in the ViewHelper variable container.

Resolves: #59255
Releases: master, 6.2
Change-Id: I6eb646bbe4c9975edff0128479e8c057395df5a1
Reviewed-on: http://review.typo3.org/30804
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>