Fix SableCCTask.maxHeapSize not being initialized explicitly

src/main/java/de/hhu/stups/sablecc/gradle/SableCCTask.java

@@ -31,6 +31,7 @@ public class SableCCTask extends SourceTask {
 		super();
 		
 		this.sableCCClasspath = null;
+		this.maxHeapSize = null;
 		this.destinationJavaDir = this.getProject().getObjects().directoryProperty();
 		this.destinationResourcesDir = this.getProject().getObjects().directoryProperty();
 	}