(not-empty coll)
If coll is empty, returns nil, else coll
user=> (not-empty [1])
[1]
user=> (not-empty [1 3 5])
[1 3 5]
user=> (not-empty [])
nil
user=> (not-empty '())
nil
user=> (not-empty {})
nil
user=> (not-empty nil)
nil
;; Same behaviour for strings
user> (not-empty "hello")
"hello"
user> (not-empty "")
nil