initial value must be set for variables (#1672)

plugins/org.yakindu.sct.generator.genmodel/src/org/yakindu/sct/generator/genmodel/SGen.xtext

@@ -25,7 +25,7 @@ GeneratorModel returns GeneratorModel:
 
 PropertyDefinition returns PropertyDefinition:
 	{PropertyDefinition} ('var') name=ID ':' typeSpecifier=TypeSpecifier
-	('=' initialValue=Expression)?;
+	'=' initialValue=Expression;
 
 GeneratorEntry returns GeneratorEntry:
 	contentType=ID elementRef=[ecore::EObject|QID] '{'