pd.isnull(Panel) errors when Panel is empty
related #3551

``` python
import pandas as pd

pd.isnull(pd.DataFrame()) # works
pd.isnull(pd.Panel()) # IndexError: index 0 is out of bounds for axis 0 with size 0
```

While the above is contrived. I hit the error when building out a `slice(0,0)` programmatically like the following:

``` python
panel = pd.util.testing.makePanel()
np.all(pd.isnull(panel.iloc[0:0])) # IndexError: index 0 is out of bounds for axis 0 with size 0
```
