Problem: Crash when evaluating an XPath expression against a document
that has no root element.
Cause: mirror_node()’s DOCUMENT_NODE branch unconditionally mirrored
document.document_element(). But a document may lack a root element —
in which case, document_element() returns null.
Fix: Return null from mirror_node() when the document has no root
element. (The caller already maps a null mirror result to a failed
evaluation, and raises an error in that case.)
Fixes https://github.com/LadybirdBrowser/ladybird/issues/10004
