Azure Monitor Scaler will panic if the timeseries has no data

[jsturtevant]

The metric scaler incorrectly checks for nil and does not check length so end up with the following exception:

 1 runtime.go:71] Observed a panic: runtime error: index out of range

This occurs if Azure monitor returns no timeseries as is sometimes the case for IncomingMessages for EventHub metrics.

This was originally discovered in Azure Monitor project: Azure/azure-k8s-metrics-adapter#90

[Vishal2696]

@ahmelsayed @jsturtevant This has to be reopened as Just now I encountered this error. When I setup a scaledobject that will trigger autoscaling based on the IncomingMessages metric of the eventhub. I encountered this error when I pointed to an event hub that has been inactive for quite amount of time. However this worked properly when I pointed to an event-hub that has active message flow.
Below is the error message I got

panic: runtime error: index out of range [0] with length 0

goroutine 815 [running]:
github.com/kedacore/keda/pkg/scalers.extractValue(0xc000808140, 0x10, 0xc0006baa50, 0x24, 0xc00011485e, 0x1a, 0xc000114840, 0x12, 0xc000114853, 0xa, ...)
        keda/pkg/scalers/azure_monitor.go:127 +0x7e1
github.com/kedacore/keda/pkg/scalers.getAzureMetric(0x218df20, 0xc0007e52b0, 0x2190ec0, 0xc000555cb0, 0x0, 0x0, 0xdf8475800, 0xd18c2e2800, 0x3, 0x6fc23ac00, ...)
        keda/pkg/scalers/azure_monitor.go:108 +0x4c5
github.com/kedacore/keda/pkg/scalers.executeRequest(0x218df20, 0xc0007e52b0, 0x2190ec0, 0xc000555cb0, 0x0, 0x0, 0xdf8475800, 0xd18c2e2800, 0x3, 0x6fc23ac00, ...)
        keda/pkg/scalers/azure_monitor.go:79 +0xab
github.com/kedacore/keda/pkg/scalers.GetAzureMetricValue(0x21d8640, 0xc0009ca940, 0xc0003f62c0, 0xc000652710, 0xc0007f5e00, 0x42de7f)
        keda/pkg/scalers/azure_monitor.go:40 +0xf9
github.com/kedacore/keda/pkg/scalers.(*azureMonitorScaler).IsActive(0xc0000c0be8, 0x21d8640, 0xc0009ca940, 0x1, 0x1, 0xc000a5b200)
        keda/pkg/scalers/azure_monitor_scaler.go:159 +0x46
github.com/kedacore/keda/pkg/handler.(*ScaleHandler).handleScaleDeployment(0xc0005558c0, 0x21d8640, 0xc0009ca940, 0xc0003ee000)
        keda/pkg/handler/scale_loop.go:119 +0x11a
github.com/kedacore/keda/pkg/handler.(*ScaleHandler).handleScale(0xc0005558c0, 0x21d8640, 0xc0009ca940, 0xc0003ee000)
        keda/pkg/handler/scale_loop.go:45 +0x6b
github.com/kedacore/keda/pkg/handler.(*ScaleHandler).HandleScaleLoop(0xc0005558c0, 0x21d8640, 0xc0009ca940, 0xc0003ee000)
        keda/pkg/handler/scale_loop.go:14 +0x1d2
created by github.com/kedacore/keda/pkg/controller/scaledobject.(*ReconcileScaledObject).startScaleLoop
        keda/pkg/controller/scaledobject/scaledobject_controller.go:330 +0x35f