Handle empty models in Pyomo-HiGHS interface

[michaelbynum]

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Summary

Currently an error gets raised when attempting to solve an empty model via the Pyomo-HiGHS interface. What should happen?

See this HiGHS issues: https://github.com/ERGO-Code/HiGHS/issues/1588

[michaelbynum]

We could add empty_model to TerminationCondition?

[blnicho]

This as discussed at the Pyomo dev call and we decided to add an EmptyModel termination condition to catch this case.