Prefer `nil` for empty lists over `()` or `'()`

[Fuco1]

There are essentially 5 ways to "initialize" a list

(let (results)
  (--each ... (push x results)))

(let ((results))
  (--each ... (push x results)))

(let ((results nil))
  (--each ... (push x results)))

(let ((results ()))
  (--each ... (push x results)))

(let ((results '()))
  (--each ... (push x results)))

Personally I use 2 and 3 and prefer 2 if there are multiple bindings at the same time

(let ((x 1)
      (y 2)
      (results)
      (z 3))
  (--each ... (push x results)))

Having it wrapped in the parens sort of signifies to me it's a list. A nil there wouldn't hurt though, I'm just lazy.

I was hoping there would be some way to distinguish () and nil for example for the purpose of analysis but they both appear as nil to the reader, so meh.

[bbatsov]

I generally go with option 2, as then it's easy to add more non-empty elements to the bindings, but I agree that 3 & 4 definitely read better. I'm fine with suggesting the use of nil, although I don't think we should discourage ().