Make valuation class more robust to null underlying TheoryEngine. (#4864)

Make valuation class more robust to null underlying TheoryEngine. (#4864)

In some use cases (unit tests, old proofs infrastructure), we use a Theory with no associated TheoryEngine. This PR makes the Valuation class more robust to this case.

This includes making the "unevaluated kinds" a no-op in this case (this is necessary for Theory::finishInit with no TheoryEngine) and adding some assertions to cases that the Theory should never call without TheoryEngine.

This is required for a new policy for dynamically configuring equality engine infrastructure in Theory.