Kate: use cmMakefile::GetSafeDefinition() for a not-required variable

...otherwise the generator failed in embedded projects via
externalproject(), since there CMAKE_KATE_MAKE_ARGUMENT is not set.
Using GetSafeDefinition() is good enough to protect against
NULL-pointers.

Alex
Merge topic 'KateDontRequireVariable' into next

e02cdba0 Kate: use cmMakefile::GetSafeDefinition() for a not-required variable