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.

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>