Check_MK Werk 5273: Fix display of CustomGraphs for metrics without current performance data

ID: 5273 Title: Fix display of CustomGraphs for metrics without current performance data Component: metrics Level: 1 Class: Bug fix Version: 1.5.0i3 The graphing system was unable to display metrics within a custom graph if the corresponding performance-data is currently not available. This would for example be the case if a check isin a temporary "Unknown" state. The rendering now correctly relies on the corresponding historical metric data instead of discarding the metric completely.