BUG: Make SCM test more robust

If the variable is empty, there is a CMake error that it can't compare
strings.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22134 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Don't scroll comboboxes that are inside a scroll area

Using the mouse in a scroll area could inadvertently change combobox
current index (on mouse over).
For conveniency, all the qMRMLNodeComboBoxes won't scroll in scrollarea by
default.
Closes #1820, #1462

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22135 3bd1e089-480b-0410-8dfb-8563597acbee