pd.isnull(Panel) errors when Panel is empty

[dalejung]

related #3551

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:

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

[jreback]

I think once #3551 this will work (and i think its 'wrong' or at least inefficient now)

[jreback]

yep...fixed in #3551