I think the joke here is that javascript has a long history of thinking that this:

    null

is the same as this:

    "null"

...Which it isn't.

Thought it was a typo on my part... but HN seems to be trimming running spaces.

HN isn't: your browser is. Runs of whitespace are collapsed in HTML to single spaces unless specifically styled otherwise. You can verify that they're there by viewing the page source.

I like to use non-breaking spaces for that kind of thing, although they still might collapse that? At least it doesn't wrap non-breaking spaces.

No, they wouldn't collapse that: non-breaking spaces aren't considered whitespace in HTML. The side effect of that is that if you copy some text with non-breaking spaces, you'll get non-breaking spaces, so something that looks as if it contains spaces won't necessarily behave as such. In HN, if you need to quote something where space is significant, you're best off formatting it as a preformatted block by indenting it:

    Here is    some    preformatted text.

It might break up the flow of the text, but if something like whitespace is significant, that's probably a good thing.