[validator] not required and empty fields should not be validated by domain

[durandx]

Example: field with domain DO_EMAIL not required, the email validator is still applyed.
Its value is "" but the check is

        if (property.value === undefined || property.value === null) {
          return true;
        }
        return emailValidation(property.value, validator.options);

https://github.com/KleeGroup/focus/blob/24a5a975b949fed3eb3d491235e4cbf67e741e3b/src/definition/validator/validate.js#L54

[pierr]

Not required and empty fields.

[pierr]

Two things:

• the value '' shoud be undefined when the form is read
• the validator should't deal with empty cases, it should not be validated when a field is undefined