Corner case: When all entries in the matrix are missing, hungarian throws an error

[scheinerman]

Thanks for this package. I stumbled across a corner case that should be easy to handle. If all entries in the matrix are missing, then hungarian throws an error:

julia> A
3×3 Matrix{Union{Missing, Int64}}:
 missing  missing  missing
 missing  missing  missing
 missing  missing  missing

julia> hungarian(A)
ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer