fix: Key mismatch for registry URL in ModifyContainerRegistry mutation

[rapsealk]

This PR is a follow-up of #1471 and resolves #1808.

An issue has been identified in the ModifyContainerRegistry mutation, where the url field was not updating as intended. The problem stemmed from a key mismatch: while the GraphQL input field uses url, the etcd uses an empty string ("") for the registry URL. This update addresses the issue by modifying the key from url to an empty string ("") if it exists.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[lizable]

LGTM.