feat: GCP Pub/Sub scaler add configurable fallback value when no metric value found

[Caislear]

Added an optional valueIfNull configuration value to the GCP Pub/Sub as also found in the older GCP Stackdriver scaler. This allows for a default fallback value when GCP returns no value for a given metric. This prevents excessive error logging especially when using multiple pubsub scalers with low subscription traffic.

Checklist

• [x] When introducing a new scaler, I agree with the scaling governance policy
• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [ ] Tests have been added
• [x] Changelog has been updated and is aligned with our changelog requirements
• [ ] A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• [x] A PR is opened to update the documentation on (repo) (if applicable)
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)

Relates to # https://github.com/kedacore/keda/issues/5896

Documentation update

[zroubalik]

/run-e2e gcp Update: You can check the progress here