check empty: true in string rule. Fixes #283

[icebob]

const schema = {
	status: {
		type: "string",
		uppercase: true,
		enum: ["a", "b", "c"],
		empty: true
	}
};

const check = v.compile(schema);
const obj = { status: "" };

console.log(check(obj), obj);

[erfanium]

I don't know why empty strings should be a valid value for enums when we can use null instead. btw the code looks good to me

[erfanium]

@icebob Also make sure schema.empty is false by default. this behavior should not be the default behavior for enums