punctuation - Closed, open, or hyphenated form for "null-space"

What is the correct spelling of the word "null-space"?

Merriam-Webster puts it in a hyphenated form "null-space", (link).

Wikipedia (link) and MathWorld (link) both put it in either open or closed form "null space" or "nullspace".

Firefox's built-in spellchecker knows only "null space" and "null-space".

I've seen all three spellings in the existing scientific literature.

What would be the correct spelling according to grammar? Is there any difference if we are talking about AmE or BrE? Or should I just pick one spelling and use it across the whole article?

Answer

null space

This is what I have always seen, and I did a quick check on google scholar just now, and verified that this is more common than the other two.