Fix Proxmox livestats 500 error with null handling

[jaycollett]

Summary

Fixes a 500 error that occurs when viewing the dashboard with a Proxmox integration configured. The test() method succeeds but livestats() crashes due to missing null checks.

Root cause: The livestats() method doesn't handle cases where API calls return null (connection failures, insufficient permissions, or malformed responses), causing fatal errors when accessing properties on null.

Changes

• Add null check in apiCall() before calling ->getBody() on potentially null response
• Check nodeData for null before passing to array_map()
• Add null checks for node_status, vm_stats, and container_stats before accessing properties
• Track valid_nodes count to prevent division by zero when calculating CPU average
• Use null coalescing (??) for optional properties like cpu and memory
• Add check to prevent division by zero when memory_total is 0
• Return "inactive" status gracefully instead of crashing
• Pass default $inactiveData array with zero values to getLiveStats() for inactive returns - the view template requires these variables to render

Testing

• Tested with valid API token and proper permissions - stats display correctly
• Tested with connection failures - gracefully shows inactive instead of 500 error
• Tested with partial data - handles missing properties without crashing
• Tested inactive status rendering - view displays zeros instead of undefined variable errors

Related Issue

Fixes linuxserver/Heimdall#1533

[mvdkleijn]

Sorry for the delay. I was busy working on https://opencommit.eu related stuff. Merging.