Fix Swap endianness data format validation

[marko1olo]

Summary

Fix Swap endianness so an invalid or empty data format argument produces a controlled operation error instead of falling through with the raw input string and later throwing word.push is not a function.

This covers the issue reproduction from #2495, where a deep link can pass an empty first argument: Swap_endianness('',2,true).

Test plan

• npx grunt configTests
• node --no-warnings --no-deprecation --openssl-legacy-provider tests/operations/index.mjs -> TOTAL 1962, PASSING 1962
• npx eslint src/core/operations/SwapEndianness.mjs tests/operations/index.mjs tests/operations/tests/SwapEndianness.mjs
• git diff --cached --check before commit

Note: the first npm ci attempt hit a network ECONNRESET, and a retry timed out locally. The dependencies installed far enough for the targeted lint, config generation, and full operation test suite above to pass.

Fixes #2495.

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