Fix undefined decimal type serialization.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@sharkitek/core",
-	"version": "2.1.1",
+	"version": "2.1.2",
 	"description": "Sharkitek core models library.",
 	"keywords": [
 		"sharkitek",

src/Model/Types/DecimalType.ts

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