Use seq for checking empty collections

* In Clojure only `false` and `nil` are falsey values
* Using seq on an empty list returns nil, which can be used for
  checking in an `if` call, Reference:

  > the use of seq as a terminating condition is the idiomatic
  > way to test whether a sequence is empty.

  The Joy of Clojure - http://www.manning.com/fogus2/