When adding alerting rule, provide a default namespace label

[catherineluse]

This PR addresses #6478 by modifying the default values for a new alerting rule so that it now creates a new label for namespace=default.

To test this PR,

1. I installed monitoring.
2. In the side nav of Cluster Explorer, I went to Monitoring > Advanced > PrometheusRules.
3. Clicked Add Alert.
4. Confirmed that in the form that appears, a default namespace=default label is there.

5. Created the resource and confirmed the YAML was saved properly:

spec:
  groups:
    - name: rancher-qa
      rules:
        - alert: rancher-qa
          expr: vector(1)
          for: 0s
          labels:
            namespace: default
            severity: none

6. Confirmed the saved value renders correctly in the detail view

[mantis-toboggan-md]

Pr looks good, I'm just being nitpicky: is there a reason to include ...this.value.rules when the array is empty?

[catherineluse]

No, this is a mistake. If the rule has already been created without any alerts, the default one shouldn't be added when it is edited. will fix