No loads causes objective function to crash

[noahrhodes]

If there are no loads in a network (e.g. they have been removed because PowerModels filters status codes != 1) then this line crashes: https://github.com/lanl-ansi/PowerModelsMLD.jl/blob/0a6c676c8d832e10d4c8f1c031f5d4b1758c7b28/src/core/objective.jl#L18

because ref(pm, n, :load) is an empty collection.