fix(sidebar): prevent null responses from sidebar API endpoints

[DHRUVI5674]

Fixes #235

Summary

Sidebar API endpoints sometimes returned null when GitHub API calls failed or returned unexpected data. This caused the Mattermost webapp sidebar to crash.

Changes

• Return empty slices instead of nil in getUnreadsData
• Ensure getSidebarData returns default SidebarContent on error
• Added safety checks in sidebar API response
• Improved JSON response handling to avoid null values

Testing

• Tested locally with GitHub API failure simulation
• Verified sidebar loads without crashing
• Confirmed empty data shows correctly

Result

The sidebar no longer crashes when API endpoints return null or fail unexpectedly.

Type of change

• Bug fix
• New feature
• Breaking change

[mattermost-build]

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