extractMandatoryLogic() fix

#607
(cherry picked from commit d1c0fb0)

src/main/java/de/metas/ui/web/window/descriptor/factory/standard/GridTabVOBasedDocumentEntityDescriptorFactory.java

@@ -634,9 +634,7 @@ private static boolean extractAlwaysUpdateable(final GridFieldVO gridFieldVO)
 
 	private static ILogicExpression extractMandatoryLogic(@NonNull final GridFieldVO gridFieldVO)
 	{
-		final ILogicExpression mandatoryLogic = gridFieldVO.getMandatoryLogic();
-
-		return mandatoryLogic.isNullExpression() ? ConstantLogicExpression.of(gridFieldVO.isMandatory()) : mandatoryLogic;
+		return gridFieldVO.isMandatoryLogicExpression() ? gridFieldVO.getMandatoryLogic() : ConstantLogicExpression.of(gridFieldVO.isMandatory());
 	}
 
 	private final void collectSpecialField(final DocumentFieldDescriptor.Builder field)