Bug fix for recent changes in moment.utc()

#1007 created a new bug when passing empty strings or `null` into `moment.utc()`:

```javascript
moment('')      //=> null
moment.utc('')  //=> TypeError: Cannot call method 'utc' of null
```

The quick fix is to see if `makeMoment()` returns `null` before trying to call `.utc()` on it.