Make separate spec cases for filter, map, etc. to handle an empty sequence

[elenam]

Babel specs for filter, map, and possibly other functions allow the first argument to be any to allow the case of an empty sequence (legal in Clojure):

(filter 5 [])

This returns an empty sequence.

However, it leads to a ClassCastException error, not a spec error, on the first argument not being a function when the sequence is not empty, for example:

(filter 5 [1])

We need a special spec case for an empty sequence since otherwise error messages for a very common error are not sufficiently descriptive.