helm: fix nil pointer evaluating

[cyclinder]

Ⅰ. Describe what this PR did

helm template higress helm/core   
Error: template: higress-core/templates/deployment.yaml:45:6: executing "higress-core/templates/deployment.yaml" at <include "gateway.podTemplate" $>: error calling include: template: higress-core/templates/_pod.tpl:128:15: executing "gateway.podTemplate" at <include "skywalking.enabled" .>: error calling include: template: higress-core/templates/_helpers.tpl:100:106: executing "skywalking.enabled" at <.Values.tracing.skywalking.service>: nil pointer evaluating interface {}.service

Use --debug flag to render out invalid YAML

Ⅱ. Does this pull request fix one issue?

fixes #3532

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

helm template higress helm/core
---
# Source: higress-core/templates/controller-serviceaccont.yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: higress-controller
  namespace: default
  labels:
    helm.sh/chart: higress-core-2.2.0
    app: higress-controller
    higress: higress-controller
    app.kubernetes.io/version: "2.2.0"
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/name: higress-controller
...

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