Use More Semantically Appropriate Null Values

[GabrielKS]

At

PowerSimulations.jl/src/core/optimization_container.jl

Line 437 in f93f740

if isempty(bus_reduction_map)

,

PowerSimulations.jl/src/network_models/network_slack_variables.jl

Line 41 in f93f740

if isempty(radial_network_reduction)

, and

PowerSimulations.jl/src/network_models/powermodels_interface.jl

Line 302 in f93f740

if isempty(radial_network_reduction)

, we have special cases for when a particular dictionary is empty. The empty dictionary is being used as a null value, but nothing would be a better semantic match: we're not just trying to indicate that we should run the algorithm with zero bus reduction mappings, for example; we're trying to indicate that we should run a different version of the algorithm that takes its input from a different data structure. There are likely other occurrences of this elsewhere in the codebase.