create an isNullOrEmpty() alias function for isNull()

[ramsestom]

I use the isNull() (https://pub.dev/documentation/validators/latest/validators/isNull.html) function quite often but I find its name misleading. Could you create an isNullOrEmpty() alias function (so that it doesn't break backward compatibility for people using this function) so that calls to this function would be self explanatory and wouldn't mislead anyone reading the code. Thanks