Werk #16803: Custom graphs: Fix crash in case of missing host/service/metric

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.

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