Werk #17069: Synthetic Monitoring: Let test services go stale if no merged XML data is available

If the rebot command fails on a test node or if all attempts time out, no merged XML data is available on the Checkmk server. In this case, the corresponding plan service will report the standard message "Item not found in monitoring data" and go UNKNOWN. Before this werk, the test services behaved in the same way. As of this werk, they instead go stale, which is the intended behavior.

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