What is the proper way to check if a particular variable is null? I use v:null as a special value to indicate something in a custom function's return value.

I have tried:

let l:v = v:null
echom l:v == v:null   " Works fine
let l:v = {'file': "something"}
echom l:v == v:null   " E735: Can only compare Dictionary with Dictionary

Note for searchers: null is sometimes called None, empty or nil.