[BUGFIX] JavaScript error in Install Tool

When using toggleButton in "Test setup" a JavaScript
error is thrown because the scrolling handler can only
be used if the fixed footer is available.

Add a check to prevent this JavaScript error.

Resolves: #60577
Releases: 6.3,6.2
Change-Id: Ie7628e83280f6af032b364586cdfb6b386663d9d
Reviewed-on: http://review.typo3.org/31828
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>