Add CoalesceOrEmptySlice for slices

[takanuva15]

Hi, the CoalesceOrEmpty function is awesome, but it doesn't work for slices. Can we add a counterpart for CoalesceOrEmpty that works for nil/empty slices?

We could call it CoalesceOrEmptySlice and define it with standard generic types like the existing Coalesce:

func CoalesceOrEmptySlice[T ~[]E, E any](v ...T) (result T, ok bool) {

(Alternatively we could try removing the comparable limitation on the existing Coalesce, but I haven't tested whether that would work for nil slices)

[chg1f]

#542