I was trying to use tools.cli in conjunction with a configuration map
loaded from a file, and use clojure.core/merge to combine the results.

This didn't work because tools.cli always uses a default value of
nil, even when no default value is specified. The nil always
overrides defaults from another source.

Example before the patch:

(def my-defaults
  {:foo 1})

(merge my-defaults
       (first (clojure.tools.cli/cli
               [] ; no arguments given
               ["--foo"]))) ; no default specified
;;=> {:foo nil}

This enhancement modifies tools.cli to completely omit arguments which
have no default specified and no value given on the command line.

After the patch, the above example returns:

;;=> {:foo 1}