fix(wazuh): fall back from empty request credentials

[Moviw]

Summary

• fall back to instance secrets when proto3 request credentials decode as empty strings
• preserve explicit non-empty request credential overrides
• add a focused regression test for the empty-request path

Closes #506

Integration and compliance

• Target: Wazuh Manager API 4.x (POST /security/user/authenticate, GET /agents)
• Interface source: Wazuh public REST API documentation
• Implementation: original change to the existing integration; no new SDK or dependency
• Credentials: users provide the Wazuh username and password through OctoBus instance secrets
• Rate limits / terms: no new requests or changed request frequency
• Risk: ListAgents is read-only; no high-risk device operation is added
• Test data: local mock responses only; no real credentials or customer data

Validation

• npm test -- --service-dir wazuh__siem (48 passed)
• npm run validate -- --service-dir wazuh__siem
• npm run pack:check
• task all (lint, unit/integration/e2e tests, smoke tests, static build)
• mutation check: restoring the old fallback makes the new regression test fail

The optional per-service coverage command still reports the repository baseline branch-coverage shortfall (88.35% after this change versus 88.30% before); all 48 tests pass.