add emptyval keyword to zonal

[rafaqz]

This PR adds an emptyval keyword which specifies what to return if the reduced area is empty.

@kadyb this should mostly solve your problem, although not sure this is the best way to do it.

TODO: allow a NamedTuple of emptyval for RasterStack

[asinghvi17]

LGTM assuming isempty is fast on a SkipMissingVal

[tiemvanderdeure]

Why can't we do this by passing init to the reducer?

[rafaqz]

No, missing or NaN init will always return missing or NaN.

@asinghvi17 isempty isn't fast if the area is mostly empty. Otherwise we could check and throw a nicer error where no emptyval is given. Maybe it's inconsequential enough to do that anyway, I haven't checked.