Handle zero values as absent values

[jmattheis]

When using default or the upcoming copy to an existing instance #147, a user may want to skip setting fields on a target struct when the source field has a zero value.

// goverter:converter
type Converter interface {
	// goverter:default DefaultOutput
	Convert(source Input) Output
}

func DefaultOutput() Output {
	return Output{Name: "jmattheis", Age: 42}
}

type Input struct  { Name string; Age int }
type Output struct { Name string; Age  int }

func use() {
	var c Converter
	source := Input{Name: "override", Age: 0}
	output := c.Convert(source) 
	// current behavior
	// output.Name = override, output.Age = 0
}

In Go the zero value can describe an absent value, goverter should support skipping the absent value so that the output would be actually

// output.Name = override, output.Age = 42
// 42 is from the DefaultOutput

Similarly, when converting a non nilable primitive type to a nilable type, the target should be set/stay nil when the source value is absent, e.g.

// goverter:converter
type Converter interface {
	// goverter:default DefaultOutput
	Convert(source Input) Output
}

func DefaultOutput() Output {
	return Output{Name: "jmattheis", Age: nil}
}

type Input struct  { Name string; Age int }
type Output struct { Name string; Age *int }

func use() {
	var c Converter
	source := Input{Name: "override", Age: 0}
	output := c.Convert(source)
	// wanted behavior
	// output.Name = override, output.Age = nil
}

I don't think this should be the default, so this feature should be behind a feature config flag.

Similar #97
Request #147 (comment)
Request #152

Please 👍 this issue if you want this functionality. If you have a specific use-case in mind, feel free to comment it.