async.done not being called if there are no input files.

I've added a call to async.done if the length of files is 0. This can happen in the case where there is a filter on the files based on modification date, e.g.

files:[{ expand:true,
          cwd:"js/",
          dest:"js.min/",
          src:["**/*.js"],
          filter: function() {
              /* return true here if source file is newer than the output file */
          }
      }]