Email validation doesn't work on an empty string

[halfer]

There isn't an implementation for validate for emails, so although an empty string will be correctly filtered to null, it will validate as true, which is not desirable.

Repro code:

$filter = \Filterus\Filter::factory('email');

$result = $filter->filter('');
var_dump($result); // Returns NULL, correct

$ok = $filter->validate('');
echo $ok ? 'Passed' : 'Failed'; // Incorrectly returns Passed

A solution for now is to use a chain, like so:

$filter = \Filterus\Filter::chain('email', 'string,min:1');

$result = $filter->filter('');
var_dump($result); // Returns NULL, correct

$ok = $filter->validate('');
echo $ok ? 'Passed' : 'Failed'; // Correctly returns Failed