empty string is wrongly accepted as a timestamp

[pjpsoares]

Context

• node version: v8.4.0
• joi version: 11.0.1
• environment (node, browser): node
• used with (hapi, standalone, ...): standalone
• any other relevant information:

What are you trying to achieve or the steps to reproduce ?

When you set a field as a timestamp it is accepting an empty string as a valid value.

    Joi.assert('', Joi.date().timestamp().required());

Which result you had ?

No error thrown.

What did you expect ?

Should throw an error as '' is not a valid timestamp

[Marsup]

Thanks for the report, it's fixed.

[pjpsoares]

Thank you for such a quick fix and for maintaining this!