update R.isEmpty to return true for 0, -0, and NaN

The current semantics offend me: it's nonsensical to ask whether a number is empty, so short of throwing a TypeError the only sensible response to such a question is “no”. Only *collections* can be empty/nonempty, strictly speaking. Since `{}` is an empty collection, I argue `R.isEmpty({})` should return true. If others agree I will open a separate pull request for that change. I'd like this PR to remain focused on `0`, `-0`, and `NaN`.