[fix] Fix crash when command is empty and app has no usage

    $ bin/bender-cli
    /Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:230
                  'Cannot find commands for ' + name.magenta
                                                    ^
    TypeError: Cannot read property 'magenta' of undefined
        at loadCommand (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:230:49)
        at executeCommand (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:280:26)
        at Object.app.router.notfound (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:359:5)
        at Router.dispatch (/Users/maciej/dev/js/flatiron/node_modules/director/lib/director/cli.js:50:21)
        at /Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:87:18
        at onComplete (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/app.js:85:5)
        at Object.exports.ensure (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/features/index.js:10:10)
        at ensureFeatures (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/app.js:91:18)
        at /Users/maciej/dev/js/flatiron/node_modules/broadway/node_modules/utile/node_modules/async/lib/async.js:116:25
        at /Users/maciej/dev/js/flatiron/node_modules/broadway/node_modules/utile/node_modules/async/lib/async.js:24:16