Null documentElement handling

[bytestream]

documentElement on the DOMDocument class can be either DocumentElement|null as noted at https://www.php.net/manual/en/class.domdocument.php

CssToInlineStyles/src/CssToInlineStyles.php

Line 137 in f4f3709

$document->removeChild($htmlElement);

doesn't correctly handle when documentElement returns NULL resulting in:

Argument 1 passed to DOMNode::removeChild() must be an instance of DOMNode, null given 

Given valid HTML input, this is noticeable in libxml 2.9.12 due to a bug in libxml https://gitlab.gnome.org/GNOME/libxml2/-/issues/318