fix: unmarshal apisix/upstream field nodes be null

[chzhuo]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues

---

Bugfix

• Description
  
  I found this error in apisix-ingress-controller log

Controller sync the upstream from APISIX will set the v1.Upstream.Nodes = nil when the APISIX upstream json like "nodes": {}

we can repeat this:

package main

import (
	"encoding/json"
	"fmt"

	v1 "github.com/apache/apisix-ingress-controller/pkg/types/apisix/v1"
)

func main() {
	data := `{"node":{}}`
	var ups v1.Upstream

	err := json.Unmarshal([]byte(data), &ups)
	if err != nil {
		panic(err)
	}
	fmt.Println(ups.Nodes == nil) // output: true
}

Output should be false, but is true

[codecov-commenter]

Codecov Report

Merging #724 (4de1e05) into master (2a73216) will not change coverage.
The diff coverage is n/a.

❗ Current head 4de1e05 differs from pull request most recent head 973bd07. Consider uploading reports for the commit 973bd07 to get more accurate results

@@           Coverage Diff           @@
##           master     #724   +/-   ##
=======================================
  Coverage   32.61%   32.61%           
=======================================
  Files          65       65           
  Lines        6500     6500           
=======================================
  Hits         2120     2120           
  Misses       4136     4136           
  Partials      244      244           

---

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[tao12345666333]

Thanks!