Fix issues related to libxml2 2.12.0 (#12802)

* Avoid passing NULL to xmlSwitchToEncoding

This otherwise switches to UTF-8 on libxml2 2.12.0

* Split tests for different err

Fix issues related to libxml2 2.12.0 (#12802)

* Avoid passing NULL to xmlSwitchToEncoding

This otherwise switches to UTF-8 on libxml2 2.12.0

* Split tests for different error reporting behaviour in libxml2 2.12.0

* Avoid deprecation warnings for libxml2 2.12.0

We can't fully get rid of the parser globals as there are still APIs
that implicitly use them.

* Temporarily disable part of test for libxml 2.12.0 regression

See https://gitlab.gnome.org/GNOME/libxml2/-/issues/634

* Review fixes

* [ci skip] Update test description

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