ENH: add is_empty

[jorisvandenbossche]

Another small function I could have used for plotting (filtering out the empties after calculating the intersection with the hemisphere)

I used the s2geography available function, but actually we should probably use the cached is_empty() method on our own Geography class? @benbovy

[benbovy]

but actually we should probably use the cached is_empty() method on our own Geography class?

Yes that might be a little more efficient but not a big difference I think. For consistency, we should probably use s2geography's function to set the cached value, actually.