Fixed optional setting fields requiring a value set

[Mailaender]

#1523 introduced a regression where everything was required because I reused a validator that is put on every widget.

[eselmeister]

The EmptyStringValidator requires a null check:

Currently, ValidationStatus.ok() is returned if value is null.

@Override
public IStatus validate(Object value) {

	if(value instanceof String text && text.isEmpty()) {
		return ValidationStatus.error(ERROR);
	}
	return ValidationStatus.ok();
}

[Mailaender]

That is already checked on InputValidator, but I can add it here, too.