fix(topbar): handle empty urls array without crashing

[yogeshwaran-c]

Description

When the urls configuration option is set to an empty array ([]), the TopBar component crashes because in the render() method it only checks if(urls) before trying to access urls[this.state.selectedIndex].url. Since an empty array is truthy in JavaScript, this check passes, but then accessing index 0 of an empty array returns undefined, and accessing .url on undefined throws a TypeError.

The fix adds a .length check: if(urls && urls.length), which is consistent with the pattern already used in componentDidMount() and setSelectedUrl() within the same file.

Motivation and Context

Fixes #5663

Users may configure urls: [] or pass ?urls=[] as a query parameter. Instead of crashing, the UI should gracefully fall back to showing the URL text input.

How Has This Been Tested?

• Verified the fix matches the existing pattern used elsewhere in the same file
• When urls is [], the component now falls through to the else branch and renders the URL text input instead of crashing
• When urls has entries, behavior is unchanged

Screenshots (if appropriate):

N/A

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.