What is empty?
Welcome - empty is a bunch of tasks for WordPress plugin developers that help automate the build thanks to Apache Ant.
Why ANT not PHING?
Go ask Google that ... there are plenty of conversations elsewhere on ANT vs PHING. It's really a personal choice.
I'm not a Java Developer .. but I'm using Jenkins CI and I also use Mac, Windows & *nix so ANT was preferred
Why not Grunt.js?
Well yes, experimentation is good ... Grunt.js may fit in there somewhere if ANT tasks for minify aren't simple