`np.array_split` in `ImpactCalc.impact` fails if Hazard is empty

[peanutfun]

The error is obscure. There is no helpful error message when computing an impact on an empty Hazard object

[chahank]

Maybe this should be fixed on the level of the Hazard? Do we want to allow for empty Hazards?

[chahank]

Short-term a useful error message is of course also great.

[ankurmahiwal118028]

Hi, the issue has been unassigned in activity for over a year. I'd like to take this on — a PR is ready (clear ValueError + regression test). Happy to defer if @peanutfun is still working on it.