fix(derive-bits): support zero-length results

[harrshita123]

Summary

• allow the native ECDH backend to return an empty secret for length == 0 while continuing to reject negative lengths
• short-circuit native PBKDF2 with an empty result after validating iterations
• add shared regression coverage that runs against native, dart2js, and dart2wasm backends

Why

The browser implementation already returns an empty Uint8List for zero-bit ECDH and PBKDF2 derivations, as defined by the Web Cryptography algorithms. The FFI implementation instead rejected the same calls, making package:webcrypto behavior platform-dependent.

For ECDH, the implementation already had an empty-result branch, but its preceding length <= 0 check made that branch unreachable. For PBKDF2, an explicit zero-length guard contradicted the algorithm's empty-result step. The replacement return follows the specification's validation order and avoids a zero-sized native allocation and isolate dispatch.

This change aligns the native backend with the browser backend and keeps negative-length validation intact.

Fixes #320.

Validation

• dart format --output none --set-exit-if-changed ...
• dart analyze --fatal-warnings .
• dart test test/webcrypto_test.dart -p vm (1,441 tests)
• dart test test/webcrypto_test.dart -p chrome -c dart2js
• dart test test/webcrypto_test.dart -p chrome -c dart2wasm