Fix GH-12870: Creating an xmlns attribute results in a DOMException

There were multiple things here since forever, see the GH thread [1]
for discussion.

There were already many

Fix GH-12870: Creating an xmlns attribute results in a DOMException

There were multiple things here since forever, see the GH thread [1]
for discussion.

There were already many fixes to this function previously, and as a
consequence of one of those fixes this started throwing exceptions for a
correct use-case. It turns out that even when reverting to the previous
behaviour there are still bugs. Just fix all of them while we have the
chance.

[1] https://github.com/php/php-src/issues/12870

Closes GH-12888.

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