fix: should not return when service type is ExternalName

[ronething]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

When there are multiple services with the type of "ExternalName", it will cause error. For example:

apiVersion: v1
kind: Service
metadata:
  name: httpbin-external-domain
spec:
  type: ExternalName
  externalName: httpbin.org
---
apiVersion: v1
kind: Service
metadata:
  name: mockapi7-external-domain
spec:
  type: ExternalName
  externalName: mock.api7.ai
---
apiVersion: apisix.apache.org/v1alpha1
kind: BackendTrafficPolicy
metadata:
  name: passhost-node
spec:
  targetRefs:
  - name: httpbin-external-domain
    kind: Service
    group: ""
  - name: mockapi7-external-domain
    kind: Service
    group: ""
  passHost: node
  scheme: https
---
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: lb-route
spec:
  parentRefs:
  - name: apisix
  rules:
  - matches:
    - path:
        type: Exact
        value: /headers
    backendRefs:
    - name: httpbin-external-domain
      port: 443
      weight: 1
    - name: mockapi7-external-domain
      port: 443
      weight: 1

The incorrect configuration generated, and the nodes list only has httpbin.org:

  "upstream": {
        "type": "roundrobin",
        "scheme": "https",
        "pass_host": "node",
        "nodes": [
          {
            "port": 443,
            "host": "httpbin.org",
            "weight": 1
          }
        ],
        "labels": {
          "managed-by": "apisix-ingress-controller"
        }
      },

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first