Fix Whirlpool variant validation

[marko1olo]

Summary

• Validate the Whirlpool variant before requesting a hasher from crypto-api.
• Convert invalid direct-link/API values such as Whirlpool('', 10) into a controlled operation error instead of an uncaught backend TypeError.
• Add operation regression coverage for the empty variant case.

Fixes #2507

Checks

• node --no-warnings --no-deprecation --openssl-legacy-provider tests/operations/index.mjs
• npx eslint src/core/operations/Whirlpool.mjs tests/operations/tests/Hash.mjs
• npx grunt configTests
• git diff --check

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