[BUGFIX] Prevent notice in SelectMultipleSideBySideElement

This patch initializes a variable globally as an array to prevent a
notice when the script tries to implode the variable.

Resolves: #70420
Releases: master
Change-Id: I0d26a44ce1ff53ef9e0b8ee0086d0a773cec2c5d
Reviewed-on: http://review.typo3.org/43835


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>