Variability: Fixed check for unchanged PresenceConditions

There was the problem, that empty presence conditions (i.e. null) could
never be filled, sue to an incorrect update check.

Issue-ref: 4273
Issue-URL: af3#4273

Signed-off-by: Andreas Bayha <bayha@fortiss.org>

org.fortiss.tooling.ext.variability/src/org/fortiss/tooling/ext/variability/util/VariabilityUtils.java

@@ -176,7 +176,8 @@ public class VariabilityUtils {
 
 		final OptionalVariationPointSpecification finalSpec = spec;
 		// Change model only, if the new presence condition is different from the old one
-		if(!spec.getPresenceCondition().getStringRepresentation().equals(pcStr)) {
+		PresenceCondition oldPC = spec.getPresenceCondition();
+		if(oldPC == null || !oldPC.getStringRepresentation().equals(pc.getStringRepresentation())) {
 			ICommandStackService.getInstance().runAsCommand(elem,
 					() -> finalSpec.setPresenceCondition(pc));
 		}