Guard against empty cmake generator expressions

COMPILER_DEFINITIONS can contain cmake generator expressions that evaluate to an empty string.
gcc fails when you pass -D without an expression as an argument.

This fixes building with cmake 3.0 which adds the following expression:
$<$<NOT:$<CONFIG:Debug>>:QT_NO_DEBUG>

My fix is probably incomplete but at least kst builds again.
COMPILE_DEFINITIONS_<CONFIG> is deprecated in 3.0 so I don't expect modules adding new expressions there.