fix: empty slice are set to nil

[aimuz]

support for empty slce, Same behavior as json.Unmarshal

json.Unmarshal

%!p(logic.A={[]})
false
[]

code

type A struct {
	AA []struct {
		C string
	}
}

var c = A{}
reader := io.LimitReader(strings.NewReader(`{"AA":[]}`), 10000)
err = mapping.UnmarshalJsonReader(reader, &c)
if err != nil {
	t.Fatal(err)
}

fmt.Printf("%#p\n", c)
fmt.Println(c.AA == nil)
fmt.Println(c.AA)

before:

%!p(logic.A={[]})
true
[]

after:

%!p(logic.A={[]})
false
[]