[Feature Request] Add option (e.g. ZeroAsNull) to treat zero values equal to missing property (similiar to EmptyAsNull)

[dionysius]

Test case:

package main

import (
	"testing"

	"github.com/sters/yaml-diff/yamldiff"
	"gopkg.in/yaml.v3"
)

var yaml1 = `
intA: 2
intB: 0
intC: 0
strA: foo
strB: ""
strC: ""
`

type MyType struct {
	IntA int    `yaml:"intA"`
	IntB int    `yaml:"intB"`
	IntC int    `yaml:"intC,omitempty"`
	StrA string `yaml:"strA"`
	StrB string `yaml:"strB"`
	StrC string `yaml:"strC,omitempty"`
}

func Test_YamlDiff_Omitempty(t *testing.T) {
	t.Parallel()

	yaml1Source, err := yamldiff.Load(yaml1)
	if err != nil {
		t.Error(err)
	}

	mytype := &MyType{}

	err = yaml.Unmarshal([]byte(yaml1), mytype)
	if err != nil {
		t.Error(err)
	}

	bYaml2, err := yaml.Marshal(mytype)
	if err != nil {
		t.Error(err)
	}

	yaml2 := string(bYaml2)

	yaml2Source, err := yamldiff.Load(yaml2)
	if err != nil {
		t.Error(err)
	}

	diffs := yamldiff.Do(yaml1Source, yaml2Source, yamldiff.EmptyAsNull())
	if len(diffs) > 0 {
		for _, diff := range diffs {
			if diff.Status() != yamldiff.DiffStatusSame {
				t.Log("diff detected\n" + diff.Dump())
				t.Fail()
			}
		}
	}
}

Test Result:

--- FAIL: Test_YamlDiff_Omitempty (0.00s)
    .../main_test.go:59: diff detected
          intA: 2
          intB: 0
        - intC: 0
          strA: "foo"
          strB: ""
        - strC: ""

If one of the diff sources contains a zero value for a property and the other has this property missing a diff will be found. This makes sense by default.

Kubernetes generates optional fields with omitempty tag (just looked up a random string property). So once a yaml is produced by kubernetes (e.g. kubectl), such output will have those properties missing. It's sometimes intentional by (because $reason) , that the other diff source has this property always set, even if set to zero.

[dionysius]

Offered you a PR #31 for this issue. Please have a look and review