AK: Have JsonArray::set() change values instead of inserting values

Resolves #18618.

8134dcc changed `JsonArray::set()` to insert elements at an index
instead of changing existing elements in-place. Since no behavior
such as `Vector::try_at()` exists yet, it returns nothing.

Userland/Libraries/LibGUI/JsonArrayModel.cpp

@@ -97,7 +97,7 @@ ErrorOr<void> JsonArrayModel::set(int row, Vector<JsonValue>&& fields)
         obj.set(field_spec.json_field_name, move(fields.at(i)));
     }
 
-    TRY(m_array.set(row, move(obj)));
+    m_array.set(row, move(obj));
     did_update();
 
     return {};