Use Numpy's any to avoid error on None

Using Python3.3, I got an error when doing

```python
pcolormesh(C, norm=SymLogNorm(1.0))
```

It complained that *None* was not iterable for:

```python
if any(xticklabels):
    ...
```

I did not experience it on Python2.7, so perhaps they
changed the implementation of *any*. In any case, using
Numpy's *any* function fixes the issue because it correctly
returns *False* if the object is *None*.