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:
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: