Cannot create datarray with ticks but no labels

I'd like to use ticks without labels. At the moment that is not possible:

```
>>> DataArray([1, 2], [(None, ('a', 'b'))])
<snip>
ValueError: ticks only supported when Axis has a label
```

Well, it is possible:

```
>>> dar = DataArray([1, 2], [('tmp', ('a', 'b'))])
>>> dar.set_label(0, None)
>>> dar.axes
(Axis(label=None, index=0, ticks=('a', 'b')),)    
```
