UI: Use property split in new operator popup

This attribute conversion operator was just added.
The UI looks more consistent with property split.

source/blender/editors/geometry/geometry_attributes.cc

@@ -325,6 +325,8 @@ static int geometry_attribute_convert_exec(bContext *C, wmOperator *op)
 static void geometry_attribute_convert_ui(bContext *UNUSED(C), wmOperator *op)
 {
   uiLayout *layout = op->layout;
+  uiLayoutSetPropSep(layout, true);
+  uiLayoutSetPropDecorate(layout, false);
 
   uiItemR(layout, op->ptr, "mode", 0, nullptr, ICON_NONE);