[2.4.0] Checkmk Werk 16803 created: Custom graphs: Fix crash in case of missing host/service/metric

[//]: # (werk v2) # Custom graphs: Fix crash in case of missing host/service/metric key | value ---------- | --- date | 2024-04-24T14:21:32+00:00 version | 2.4.0b1 class | fix edition | cee component | metrics level | 1 compatible | yes Custom graphs can contain elements whose host or service is non-existant. This happens for example when a host is removed from the monitoring after one of its metrics has been added to a custom graph. In such cases, no graph was rendered. Instead, the Checkmk UI displayed the message "Cannot calculate graph recipes" and showed a traceback. As of this werk, the UI instead renders no lines for such elements and denotes them with "n/a" in the graph legend.