allowing cache.clean to be called with no args

Currently if you do this
```javascript
bower.commands.cache.clean()
```
the code errors out:
```
TypeError: Cannot call method 'map' of undefined
  at Object.clean (pathTo/bower/lib/commands/cache/clean.js:23:29)
```
which means you have to do this:
```javascript
bower.commands.cache.clean([])
```
which is a bit odd.

This PR allows `clean` to be called with no args.