Refactor Variable type and add omitted status

Reorganized methods to ensure a logical method order and added the `present` field to track omitted values in JSON. This improves clarity and functionality by correctly indicating when a value was omitted.

niljson.go

@@ -12,11 +12,20 @@ import (
 type Variable[T any] struct {
 	value   T
 	notNil  bool
+	present bool
 }
 
-// Value returns the value of the Variable
-func (v *Variable[T]) Value() T {
-	return v.value
+// IsNil returns true when a Variable is nil
+func (v *Variable[T]) IsNil() bool {
+	return !v.notNil
+}
+
+// NewVariable returns a new Variable of generic type
+func NewVariable[T any](value T) Variable[T] {
+	return Variable[T]{
+		notNil: true,
+		value:  value,
+	}
 }
 
 // NotNil returns true when a Variable is not nil
@@ -24,9 +33,9 @@ func (v *Variable[T]) NotNil() bool {
 	return v.notNil
 }
 
-// IsNil returns true when a Variable is nil
-func (v *Variable[T]) IsNil() bool {
-	return !v.notNil
+// Ommited returns true if a value was omitted in the JSON
+func (v *Variable[T]) Ommited() bool {
+	return !v.present
 }
 
 // Reset resets the value to the Variable to a zero value and sets it to be nil
@@ -36,12 +45,9 @@ func (v *Variable[T]) Reset() {
 	v.notNil = false
 }
 
-// NewVariable returns a new Variable of generic type
-func NewVariable[T any](value T) Variable[T] {
-	return Variable[T]{
-		notNil: true,
-		value:  value,
-	}
+// Value returns the value of the Variable
+func (v *Variable[T]) Value() T {
+	return v.value
 }
 
 // NilBoolean is an boolean type that can be nil
@@ -82,6 +88,7 @@ type NilString = Variable[string]
 
 // UnmarshalJSON satisfies the json.Unmarshaler interface for generic Variable types
 func (v *Variable[T]) UnmarshalJSON(data []byte) error {
+	v.present = true
 	if string(data) != "null" {
 		v.value = *new(T)
 		v.notNil = true