Werk #15305: prism_vm_stats: don't go stale if no memory usage present

Component	Checks & agents
Title	prism_vm_stats: don't go stale if no memory usage present
Date	Nov 2, 2023
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.3.0b1 Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT

State will be OK and "No memory usage data available" will be shown as summary.

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