Fix #63206: Fully support error/exception_handler stacking, even with null or inside...

Fix #63206: Fully support error/exception_handler stacking, even with null or inside the handler

Always push the current user_error/exception_handler to the stack,
even when it is empty, so restore_error_handler() always works as
expected.

The user_error_handler is especially temporarily empty when we are inside
the error handler, which caused inconsistent behaviour before.