fix(monitoring): guard against nil pointer in pooler PodMonitor template

[philippemnoel]

Summary

• Fixes nil pointer panic in podmonitor-pooler.yaml when a pooler defines monitoring.enabled: true without a podMonitor sub-key
• Go templates evaluate all arguments to and before calling the function, so .monitoring.podMonitor.enabled is evaluated even when .monitoring.podMonitor is nil, causing a nil pointer panic
• Replaces the single and check with nested if blocks so .monitoring.podMonitor.enabled is only evaluated when .monitoring.podMonitor is confirmed non-nil

How to reproduce

Define a pooler with monitoring enabled but without a podMonitor key:

poolers:
  - name: rw
    type: rw
    instances: 3
    poolMode: transaction
    parameters:
      max_client_conn: "1000"
      default_pool_size: "25"
    monitoring:
      enabled: true

This produces:

Error: template: cluster/templates/podmonitor-pooler.yaml:3:54: executing "cluster/templates/podmonitor-pooler.yaml" at <.monitoring.podMonitor.enabled>: nil pointer evaluating interface {}.enabled