feat: add in pubsub valueIfNull optional config

Signed-off-by: Benjamin Woodley <ben.woodley@y42.com>

content/docs/2.15/scalers/gcp-pub-sub.md

@@ -19,6 +19,7 @@ triggers:
     mode: "SubscriptionSize" # Optional - Default is SubscriptionSize - SubscriptionSize or OldestUnackedMessageAge
     aggregation: "sum" # Optional - Only meaningful for distribution-valued metrics
     value: "5.5" # Optional - Default is 10
+    valueIfNull: '0.0' # Optional - Default is ""
     activationValue: "10.5" # Optional - Default is 0
     timeHorizon: "1m" # Optional - Default is 2m and with aggregation 5m
     # Exactly one of the subscription or topic name options is required
@@ -42,6 +43,8 @@ The Google Cloud Platform (GCP) Pub/Sub trigger allows you to scale based on any
 
 - `timeHorizon` - Time range for which you want to retrieve the matrics. (Default: `2m` and Default with aggregation field: `5m`)
 
+- `valueIfNull` - Value returned if request returns no timeseries.(Default: `""`, Optional, This value can be a float) 
+
 - `subscriptionName` defines the subscription that should be monitored. You can use different formulas:
   - Just the subscription name, in which case you will reference a subscription from the current project or the one specified in the credentials file used.
   - Use the full link provided by Google, so that you can reference a subscription that is hosted in another project Eg: `projects/myproject/subscriptions/mysubscription`.