fix: handle missing license field after removal in Dgraph v25.0.0

[eileenaaa]

Description

Dgraph v25.0.0 removed the Enterprise license and the license field from the getState response #9367.
As a result, Ratel’s Cluster page fails to render because the frontend still attempts to access clusterState.license, which no longer exists in the API response.

This PR updates the Cluster page logic to:

• Use optional chaining and nullish coalescing to safely read license-related data.
• Render license information only when the license field exists.

These changes prevent runtime errors and allow the Cluster page to render correctly even when no license field is present.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• For public APIs, new features, etc., PR on docs repo
  staged and linked here

[matthewmcneely]

Closes https://github.com/orgs/dgraph-io/discussions/9542