feat(emby): support passwordless accounts

[zijiren233]

Summary

• Allow Emby users with an explicitly empty password to authenticate.
• Preserve password whitespace and keep missing credentials or empty API keys invalid.
• Accept passwordless credentials through the remote provider API and expose the CLI --no-password option.
• Update proto comments and CLI documentation.

Validation

• Media provider test suite: 552 passed, 5 ignored.
• Focused Emby, gRPC, CLI, formatting, clippy, proto freshness, and documentation checks passed.