Added check in autoscale_None for completely masked pcolor plots.

Producing a pcolor of a completely masked array currently breaks v1.2.x with numpy 1.6:

```
import matplotlib.pyplot as plt
import numpy as np

data = np.ma.masked_all((12, 20))
plt.pcolor(data)
plt.show()
```

With the error:

```
Traceback (most recent call last):
  File "", line 7, in <module>
    plt.pcolor(data)
  File "matplotlib/pyplot.py", line 2758, in pcolor
    ret = ax.pcolor(*args, **kwargs)
  File "matplotlib/axes.py", line 7363, in pcolor
    collection.autoscale_None()
  File "matplotlib/cm.py", line 310, in autoscale_None
    self.norm.autoscale_None(self._A)
  File "matplotlib/colors.py", line 924, in autoscale_None
    self.vmin = ma.min(A)
  File "numpy/ma/core.py", line 5879, in min
    return asanyarray(obj).min(axis=axis, fill_value=fill_value, out=out)
  File "numpy/ma/core.py", line 5056, in min
    result = self.filled(fill_value).min(axis=axis, out=out).view(type(self))
ValueError: zero-size array to minimum.reduce without identity
```

This fix checks that the size of the array is >0 before checking the minimum value in autoscale_None.