Fix error when statusBar is undefined

The package cannot load because `atom.workspaceView.statusBar` is not defined. 

Using the example in [atom/status-bar](https://github.com/atom/status-bar) :
```Coffeescript
module.exports =
  activate: ->
    atom.packages.once 'activated', ->
      atom.workspaceView.statusBar?.appendLeft('<span>hi!</span>')
```
You notice the `?`, so it means this variable could be null.