feat(parser): adds IsEmpty() method for OptNil* generated wrappers

[MikeKirillov]

Summary

When generating schemas with optional, nullable fields, it is often necessary to distinguish between a value being explicitly set to null versus being entirely omitted from the input (e.g., the "Zero" test case in the included examples).

ogen/internal/integration/json_test.go

Lines 84 to 104 in f48de28

	for _, tc := range []struct {
	Name string
	Value api.OptNilString
	Result string
	}{
	{
	Name: "Zero",
	Value: api.OptNilString{},
	Result: "{}",
	},
	{
	Name: "Set",
	Value: api.NewOptNilString("foo"),
	Result: `{"key":"foo"}`,
	},
	{
	Name: "Nil",
	Value: api.OptNilString{Null: true, Set: true},
	Result: `{"key":null}`,
	},
	} {

To simplify this check, this PR introduces the IsEmpty() method for OptNil*** wrappers, providing a clean and efficient way to verify if a field was provided.

Usage Example

Consider the following struct:

type OrderDto struct {
	OrderUUID uuid.UUID `json:"order_uuid"`
	HullUUID uuid.UUID `json:"hull_uuid"`
	EngineUUID uuid.UUID `json:"engine_uuid"`
	ShieldUUID OptNilUUID `json:"shield_uuid"`
	WeaponUUID OptNilUUID `json:"weapon_uuid"`
	// ...
}

Before

Previously, checking for an omitted value required a more verbose approach:

func OrderCreateRqToModel(orderRq *orderv1.CreateOrderRequest) model.CreateOrderRq {
    // Required fields
	result := model.CreateOrderRq{
		HullUUID:   orderRq.HullUUID.String(),
		EngineUUID: orderRq.EngineUUID.String(),
	}
    // Process optional field if present
	if orderRq.GetShieldUUID().IsSet() && !orderRq.GetShieldUUID().IsNull() {
		val := orderRq.GetShieldUUID().Value.String()
		result.ShieldUUID = &val
	}
    // Handle omitted field
	if !orderRq.GetWeaponUUID().IsSet() && !orderRq.GetWeaponUUID().IsNull() {
		// ...
	}
	return result
}

After

With IsEmpty(), the logic is much more concise:

func OrderCreateRqToModel(orderRq *orderv1.CreateOrderRequest) model.CreateOrderRq {
    // Required fields
	result := model.CreateOrderRq{
		HullUUID:   orderRq.HullUUID.String(),
		EngineUUID: orderRq.EngineUUID.String(),
	}
    // Process optional field if present
	if !orderRq.GetShieldUUID().IsEmpty() {
		val := orderRq.GetShieldUUID().Value.String()
		result.ShieldUUID = &val
	}
    // Handle omitted field
	if orderRq.GetWeaponUUID().IsEmpty() {
		// ...
	}
	return result
}