Fix GH-16808: Segmentation fault in RecursiveIteratorIterator->current() with a xml element input

When the current data is invalid, NULL must be returned. At least that's
how the check i

Fix GH-16808: Segmentation fault in RecursiveIteratorIterator->current() with a xml element input

When the current data is invalid, NULL must be returned. At least that's
how the check in SPL works and how other extensions do this as well.
If we don't do this, an UNDEF value gets propagated to a return value
(misprinted as null); leading to issues.

Closes GH-16825.

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