A textcircle with no font-size raises Undefined array key "fontsize" warnings #645

A <textcircle> with no font-size raises three PHP warnings when the document is written.

$mpdf = new \Mpdf\Mpdf(['mode' => 'c']);
$mpdf->WriteHTML('<textcircle r="30mm" top-text="Hello" />');
$mpdf->Output('', 'S');

Undefined array key "fontsize" @ src/Mpdf.php:8058
Undefined array key "fontsize" @ src/Mpdf.php:8430
Undefined array key "fontsize" @ src/Mpdf.php:8433

The text circle's object is drawn without a fontsize key when neither the tag nor CSS sets one. It should fall back to the current font size.

Reproduced on gravitypdf at e685435, on PHP 8.5. Found while working on #644.