Fix undefined date serialization.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@sharkitek/core",
-	"version": "2.1.2",
+	"version": "2.1.3",
 	"description": "Sharkitek core models library.",
 	"keywords": [
 		"sharkitek",

src/Model/Types/DateType.ts

@@ -12,7 +12,7 @@ export class DateType extends Type<string, Date>
 
 	serialize(value: Date): string
 	{
-		return value.toISOString();
+		return value?.toISOString();
 	}
 }