`optional` shouldn't stop ignore validation

[OscarGodson]

If you have a field, say phone number and you make it optional it should still validate the field if you type some value. If no value then ignore it when submitting.

Examples:

<input class="optional validate-phone-number" value="123">

That would be invalid.

<input class="optional validate-phone-number" value="1234567890">

That would be valid.

<input class="optional validate-phone-number" value="">

That would be valid.