fix: dto에서 optional 제거

Author: summersummerwhy

apps/backend/src/workspace/dtos/userWorkspace.dto.ts

@@ -1,7 +1,7 @@
 export class UserWorkspaceDto {
   workspaceId: string;
   title: string;
-  description?: string | null;
-  thumbnailUrl?: string | null;
+  description: string | null;
+  thumbnailUrl: string | null;
   role: 'owner' | 'guest';
 }