两个Map结构不同但值为nil时，decode will panic

[KellendF]

What happened:
两个Map结构不同但值为nil时，decode will panic
What you expected to happen:
返回空Map
How to reproduce it (as minimally and precisely as possible):

type UserInfo struct {
	Name    string
	Address map[string]string
	Family  map[string]int
}

func (UserInfo) JavaClassName() string {
	return "com.test.ReqInfo"
}

func TestIssueCase(t *testing.T) {
	req := &UserInfo{
		Name:    "test",
		Address: nil,
		Family:  nil,
	}

	hessian.RegisterPOJO(req)

	encoder := hessian.NewEncoder()
	err := encoder.Encode(req)
	if err != nil {
		t.Error(err)
		return
	}

	enBuf := encoder.Buffer()

	decoder := hessian.NewDecoder(enBuf)
	deReq, err := decoder.Decode()
	assert.Nil(t, err)

	t.Log(deReq)

	assert.True(t, reflect.DeepEqual(req, deReq))
}

=== RUN   TestIssueCase
--- FAIL: TestIssueCase (0.00s)
panic: reflect.Set: value of type map[string]string is not assignable to type map[string]int [recovered]
	panic: reflect.Set: value of type map[string]string is not assignable to type map[string]int

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