[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/32149
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>