Item::get_date: fix return type on unparsable date (#753)
Item::get_date: fix return type on unparsable date (#753)

The `Parse\Date::parse` method can return `false` when it fails to parse the value and `get_date` will return it as is but its PHPDoc annotation does not allow returning booleans.

Let’s fix it by converting such values to `null`.
simplepie/simplepie,simplepie/simplepie