fix: return 404 error when admin user is not found during complex creation

apps/backend/src/modules/complex/handlers/create-complex.handler.ts

@@ -25,7 +25,10 @@ export async function createComplexHandler(c: AppContext) {
       where: { email: adminEmail },
       select: { id: true },
     });
-    userId = existingUser?.id;
+    if (!existingUser) {
+      return c.json({ message: 'El usuario no existe.' }, 404);
+    }
+    userId = existingUser.id;
   }
 
   const complex = await createComplex({