Improve toObject method

When a property is either 'undefined' or 'null' it will return 'null' when using the toObject method.
Previously these values would throw an error

lib/Model.js

@@ -200,7 +200,11 @@ class Model
 			{
 				let value = this.get(key)
 
-				if (value.toObject)
+				if (typeof value === 'undefined')
+				{
+					value = null
+				}
+				else if (value && value.toObject)
 				{
 					value = value.toObject()
 				}

test/model-to-object.js

@@ -39,6 +39,26 @@ describe('Model toObject', () =>
 		expect(object).toEqual({ name: 'foo' })
 	})
 
+	it('returns an object with specified key when null', () =>
+	{
+		const instance = new Model({
+			name: null
+		})
+
+		const object = instance.toObject('name')
+
+		expect(object).toEqual({ name: null })
+	})
+
+	it('returns a null value for specified key when undefined', () =>
+	{
+		const instance = new Model()
+
+		const object = instance.toObject('name')
+
+		expect(object).toEqual({ name: null })
+	})
+
 	it('returns an object with specified keys', () =>
 	{
 		const instance = new Model({