Add isEmpty on required string

[morrowyn]

ValidationBuilder().required("").build() only works on strings that are null. However when a string is empty "" the validator says that it's valid. I expected an invalid error.

[TheJosh]

Yeah, it appears that when using a TextEditingController, the value is often silently converted from a null into an empty string, so as a result this required check never hits, even though the fields are empty.

Workaround is adding minLength(1) but this is a bit silly and also gives a not very helpful error message.

[TheJosh]

Another workaround is an extension method:

extension CustomValidationBuilder on ValidationBuilder {
  notEmpty() => add((value) {
    return value.isEmpty ? 'This field is required' : null;
  });
}