feat(gcp-stackdriver): adding valueIfNull (#1285)

Signed-off-by: VAN KELST Baptiste <[email protected]>

content/docs/2.13/scalers/gcp-stackdriver.md

@@ -17,6 +17,7 @@ triggers:
     projectId: my-project-id
     filter: 'metric.type="storage.googleapis.com/network/received_bytes_count" AND resource.type="gcs_bucket" AND metric.label.method="WriteObject" AND resource.label.bucket_name="my-gcp-bucket"'
     targetValue: '100.50'
+    valueIfNull: '0.0' #Optional - Default is ""
     activationTargetValue: "10.5" # Optional - Default is 0
     credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON
     alignmentPeriodSeconds: '60'
@@ -30,6 +31,7 @@ triggers:
 - `filter` - The stackdriver query filter for obtaining the metric. The metric is for the last minute and if multiple values are returned, the first one is used.
 - `targetValue` - Average target value to trigger scaling actions. (Default: `5`, Optional, This value can be a float)
 - `activationTargetValue` - Target value for activating the scaler. Learn more about activation [here](./../concepts/scaling-deployments.md#activating-and-scaling-thresholds).(Default: `0`, Optional, This value can be a float)
+- `valueIfNull` - Value return if request return no timeseries.(Default: `""`, Optional, This value can be a float) 
 
 The `credentialsFromEnv` property maps to the name of an environment variable in the scale target (`scaleTargetRef`) that contains the service account credentials (JSON). KEDA will use those to connect to Google Cloud Platform and collect the configured stack driver metrics.