test(api): match UserControllerTest with new behavior

the `ageRestriction` DTO now omits fields instead of having them be null
so this updates the test to check via `doesNotExist()`

komga/src/test/kotlin/org/gotson/komga/interfaces/api/rest/UserControllerTest.kt

@@ -113,7 +113,7 @@ class UserControllerTest(
           jsonPath("$.sharedLibrariesId") { doesNotExist() }
           jsonPath("$.labelsAllow") { isEmpty() }
           jsonPath("$.labelsExclude") { isEmpty() }
-          jsonPath("$.ageRestriction") { isEmpty() }
+          jsonPath("$.ageRestriction") { doesNotExist() }
         }
     }
   }