fix(simplex): support zero objectives after phase one

[rasheedja]

Summary

• Fix empty Max and Min objectives after phase one introduces artificial variables. For example, optimizing a zero objective subject to x >= 1 previously raised the empty-list exception in foldVarLitMap.
• Use Data.Map.unionsWith (+) to sum coefficient maps with the empty map as the zero expression. Replace the related custom two-map merge with Data.Map.unionWith (+), using the existing dependency.
• Add solver regressions that check the zero optimum and feasibility of the returned assignment, and update the empty-fold unit test.

This PR targets master and has no dependency on the other review fixes.

Validation

• Both new solver regressions failed before the fix.
• stack test --no-terminal: 273 examples, 0 failures.
• cabal check: no errors or warnings.
• Fourmolu 0.15.0.0 check of all tracked Haskell files passed.
• stack haddock --no-haddock-deps and git diff --check passed.