fix(helm): harden dnsConfig helper per review

Address CodeRabbit feedback on the dnsConfig helper:

- When dnsPolicy is "None", Kubernetes requires at least one nameserver
  (searches/options alone are rejected by the API server). Tighten the
  guard to require .nameservers specifically instead of any config field.
- Tolerate a null/missing dnsConfig object (`and . .enabled`) and default
  .policy to "ClusterFirst" when unset, so the template never panics or
  emits an empty `dnsPolicy: ""`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: DrFaust92

helm/robusta/templates/_helpers.tpl

@@ -18,12 +18,13 @@ Pass the component's dnsConfig dict as the context, e.g.:
 When disabled (or unset) it falls back to dnsPolicy: ClusterFirst.
 */}}
 {{- define "robusta.dnsConfig" -}}
-{{- if .enabled -}}
+{{- if and . .enabled -}}
+{{- $policy := default "ClusterFirst" .policy -}}
 {{- $hasConfig := or .nameservers .searches .options -}}
-{{- if and (eq .policy "None") (not $hasConfig) -}}
-{{- fail "dnsConfig: when dnsPolicy is 'None', you must set at least one of nameservers, searches, or options" -}}
+{{- if and (eq $policy "None") (not .nameservers) -}}
+{{- fail "dnsConfig: when dnsPolicy is 'None', you must provide at least one nameserver (Kubernetes requires it)" -}}
 {{- end -}}
-dnsPolicy: {{ .policy | quote }}
+dnsPolicy: {{ $policy | quote }}
 {{- if $hasConfig }}
 dnsConfig:
   {{- with .nameservers }}