This PR adds unit tests for EmptyExec and DynProofPlan to improve the overall test coverage of the proof-of-sql crate.

• Added crates/proof-of-sql/src/sql/proof_plans/empty_exec_test.rs
• Added crates/proof-of-sql/src/sql/proof_plans/dyn_proof_plan_test.rs
• Registered the new test modules in crates/proof-of-sql/src/sql/proof_plans/mod.rs

Closes #560

/claim #560