Make externalClusters conditional to prevent null value in standalone mode

[philippemnoel]

Summary

Fixes the spec.externalClusters: Invalid value: "null" error that occurs when deploying in standalone mode with backups disabled and using strict YAML validation (server-side apply).

This fix aligns with upstream CloudNativePG charts PR #760 to minimize future rebase conflicts.

Root Cause

When mode: standalone and backups.enabled: false, the generated YAML ends with:

externalClusters:

with no content. Strict YAML parsers interpret this as externalClusters: null instead of an empty array, causing CNPG operator validation to fail.

Changes Made

1. Fixed _external_clusters.tpl

• Moved externalClusters: field inside conditional logic
• Standalone mode: only output field if custom clusters defined
• Recovery/replica modes: always output field with generated config
• Matches upstream approach while preserving ParadeDB's custom externalClusters feature

2. Updated CI Validation

• Added --server-side --validate=strict flags to kubectl apply in deploy-cluster action
• Ensures future CI runs catch similar YAML validation issues

Testing

• Verified fix works in standalone mode with backups disabled
• Confirmed recovery and replica modes still work correctly
• CI tests pass with stricter validation

Related

• Upstream issue: cnpg/cluster - spec.externalClusters not an array in cluster.yaml #753
• Upstream fix: fix(template): standalone mode should skip externalClusters #760