Use More Semantically Appropriate Null Values

At https://github.com/NREL-Sienna/PowerSimulations.jl/blob/f93f740f8d4ffc61895bf3a8f59f400fdc3961a0/src/core/optimization_container.jl#L437, https://github.com/NREL-Sienna/PowerSimulations.jl/blob/f93f740f8d4ffc61895bf3a8f59f400fdc3961a0/src/network_models/network_slack_variables.jl#L41, and https://github.com/NREL-Sienna/PowerSimulations.jl/blob/f93f740f8d4ffc61895bf3a8f59f400fdc3961a0/src/network_models/powermodels_interface.jl#L302, 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.

Start with the empty-dictionary cases in src/core/optimization_container.jl, src/network_models/network_slack_variables.jl, and src/network_models/powermodels_interface.jl. Trace how each dictionary is used as a null value, search the codebase for similar occurrences, and verify that the refactor distinguishes an absent input from an empty mapping without changing ordinary behavior.