fix(charts): Ensure explicit empty externalClusters list for standalone mode

When the cluster is in 'standalone' mode, the 'externalClusters' key was
rendered with an implicitly empty value. This commit explicitly defines
'externalClusters: []' for 'standalone' mode to ensure valid YAML and
clarity.

Additionally, the 'externalClusters:' key has been relocated within the
'recovery' and 'replica' mode blocks to maintain consistent YAML list
formatting across all operational modes.

Signed-off-by: Enrique Hernández Bello <ehernandez@maarlab.com>

Author: ehbello

charts/cluster/templates/_external_clusters.tpl

@@ -1,7 +1,8 @@
 {{- define "cluster.externalClusters" -}}
-externalClusters:
 {{- if eq .Values.mode "standalone" }}
+externalClusters: []
 {{- else if eq .Values.mode "recovery" }}
+externalClusters:
   {{- if eq .Values.recovery.method "pg_basebackup" }}
   - name: pgBaseBackupSource
      {{- include "cluster.externalSourceCluster" .Values.recovery.pgBaseBackup.source | nindent 4 }}
@@ -16,6 +17,7 @@ externalClusters:
       {{- include "cluster.barmanObjectStoreConfig" $d | nindent 4 }}
   {{- end }}
 {{- else if eq .Values.mode "replica" }}
+externalClusters:
   - name: originCluster
   {{- if not (empty .Values.replica.origin.objectStore.provider) }}
     barmanObjectStore: