text-outline-width without a colour raises Undefined array key "outline-COLOR" #650

text-outline-width without a text-outline-color raises a PHP warning when the document is written:

$mpdf = new \Mpdf\Mpdf(['mode' => 'c']);
$mpdf->WriteHTML('<p style="text-outline-width: 0.2mm">x</p>');
$mpdf->Output('', 'S');

Undefined array key "outline-COLOR" @ src/Mpdf.php:9871, in SetTextOutline()

With no colour given, the outline should take a default colour, not read a key that was never set. In CSS terms that default is currentColor, which #540 is adding.

Reproduced on gravitypdf at e685435, on PHP 8.5. Found while working on #648. Part of #552.