Fix BLAKE2 output encoding validation

[marko1olo]

Fixes #2510.
Fixes #2511.

Summary

• throw OperationError for unsupported BLAKE2b/BLAKE2s output encodings instead of returning it as operation output
• avoid the follow-on Data is not a valid string: {"type":"OperationError"} error
• add regression coverage for empty output encoding values for both operations

Checks

• node --no-warnings --no-deprecation --openssl-legacy-provider tests/operations/index.mjs -> 1962 passing
• npx eslint src/core/operations/BLAKE2b.mjs src/core/operations/BLAKE2s.mjs tests/operations/tests/BLAKE2b.mjs tests/operations/tests/BLAKE2s.mjs
• npx grunt configTests
• git diff --check

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