It's a word that starts with a $ and is interpreted in mIRC. Sounds like an identifier to me. Yes $null()'s usage isn't explained in the help file, though is it relevant? There are numerous things unmentioned in the help file that do exist in the scripting language. Seeing as how $null() acts the way you would expect it to do, and it does so consistently, it is fair to assume it was created with the intention to also work as a $noop() although of course only Khaled can clarify this.
No. As far as mIRC is concerned, $null() doesn't exist
. It is not a built-in identifier, and therefore what you said to FaiNT is completely besides the point. The fact that it does what you expect, is only a side effect of the fact that an identifier that doesn't exist, returns nothing - again, if you turn on Identifier Warning in remote, then you will see that its use generates a warning.