Resolve TODOs in ext/dom around nullable content (#14999)

It's indeed possible this is NULL. When you create a new text-like node
in libxml and pass NULL as content, you do get NULL in t

Resolve TODOs in ext/dom around nullable content (#14999)

It's indeed possible this is NULL. When you create a new text-like node
in libxml and pass NULL as content, you do get NULL in the content field
instead of the empty string. You can hit this by creating DOMText or
DOMComment directly and not passing any argument. This could also be
created internally.
We refactor the code such that this detail is hidden and we add a test
to check that it correctly throws an exception.

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