valFn = function (v) {
            if (v == null) return true;
            return (typeof v === "string");
        }
Returns a standard string dataType Validator.
    @example
        // Assume em1 is a preexisting EntityManager.
        var custType = em1.metadataStore.getEntityType("Customer");
        var regionProperty - custType.getProperty("Region");
        // Validates that the value of the Region property on Customer is a string.
        regionProperty.validators.push(Validator.string());
    @method string
    @static
    @return {Validator} A new Validator