Graphite Scaler doesn't properly handle null responses

[bpinske]

Report

In some configurations, it's possible for Graphite to return null for its latest datapoint. This gets erroneously interpeted as a zero.

curl 'http://graphite' 2> /dev/null |jq
[
  {
    "target": "sumSeries(metric)",
    "tags": {
      "name": "metric",
      "aggregatedBy": "sum"
    },
    "datapoints": [
      [
        15323331,
        1651092420
      ],
      [
        null,
        1651092480
      ]
    ]
  }
] 


kubectl get --raw '/apis/external.metrics.k8s.io/v1beta1/namespaces/namespace/metric?labelSelector=app=app' | jq
{
  "kind": "ExternalMetricValueList",
  "apiVersion": "external.metrics.k8s.io/v1beta1",
  "metadata": {},
  "items": [
    {
      "metricName": "metric",
      "metricLabels": null,
      "timestamp": "2022-04-27T20:47:57Z",
      "value": "0" # <<< mis-interpreting null as zero
    }
  ]
} 

Expected Behavior

Nulls are tolerated with the most recent non-null datapoint being used instead.
Nulls should not be mis-interpeted as a zero value

Actual Behavior

Null graphite responses cause apps to erroneously scale down.

Steps to Reproduce the Problem

1. Create a graphite scaler
2. Allow the query to return a null value
3. Value is considered as a zero

Logs from KEDA operator

No response

KEDA Version

No response

Kubernetes Version

No response

Platform

No response

Scaler Details

No response

Anything else?

No response