fix: `fix-null-undefined-serialization` doesn't replace undefined with null

[Rudomitori]

For some reason, NSwag started producing a space between undedined and ;, so the old pattern doesn't match anymore.

In several files I've saw the comment:

{% comment %}space before the ';' is important, so that 'undefined' here is not replaced with null{% endcomment %}

But in my project I haven't noticed any unexpected replacing of undefined with null after these changes.