CCM-11343: fix typecheck

Author: bhansell1

lambdas/backend-api/src/templates/infra/template-repository.ts

@@ -61,6 +61,8 @@ const letterAttributes: Record<keyof LetterProperties, null> = {
   owner: null,
   personalisationParameters: null,
   templateType: null,
+  proofingEnabled: null,
+  supplier: null,
 };
 
 export class TemplateRepository {