[2.3.0] Checkmk Werk 16801 created: Custom graphs: Fix crash in case of unavailable scalars

[//]: # (werk v2) # Custom graphs: Fix crash in case of unavailable scalars key | value ---------- | --- date | 2024-04-23T10:22:44+00:00 version | 2.3.0b7 class | fix edition | cee component | metrics level | 1 compatible | yes When adding a scalar to a custom graph, it is possible that no value is available for this scalar. For example, this is the case when adding the CRIT threshold of a metric for which no thresholds are configured. In such cases, no graph was rendered. Instead, the Checkmk UI displayed the message "Cannot calculate graph recipes" and showed a traceback. This werk restores the correct behavior: No lines are rendered for unavailable scalars and they are denoted with "n/a" in the graph legend.