Werk #16801: Custom graphs: Fix crash in case of unavailable scalars

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.

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