Add an EmptyValidator

[elpete]

This validator is useful when a field is not required but if it exists it cannot be empty.
This is needed since type validators ignore empty values as valid, but an empty string is not a date!

Here's why I added it:

{
    "startDate": {
        "required": false,
        "type": "date"
    }
}

These validation constraints are typical in our search endpoint — no keys are needed, but if it exists it needs to be a date. Unfortunately, required counts empty strings as valid and type ignores empty strings as "not required". Then my application is dealing with an empty string trying to use it as a date.

With this new constraint, I would add it in like so:

{
    "startDate": {
        "required": false,
        "empty": false,
        "type": "date"
    }
}

It still isn't required, but the value must be a non-empty value and it must be parseable as a date.

[lmajano]

@elpete can you please send me a PR for the docs on this please?