tests: Treat empty SECP2561_TEST_ITERS as if it was unset
tests: Treat empty SECP2561_TEST_ITERS as if it was unset

bitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,peterdettman/secp256k1,peterdettman/secp256k1