[FIX] web: correctly position carret in list

In list view, the carret was not correctly positioned in empty cells (i.e.
handle widget in readonly).

This rev. ensures that the position rules are only applied if the cell is not
empty.

Fixes odoo#23984

addons/web/static/src/less/list_view.less

@@ -37,12 +37,14 @@
         .user-select(none);  // Prevent unwanted selection while sorting
 
         &::after {
-            margin-left: 6px;
-            position: absolute;
             font-family: FontAwesome;
             content: "\f0d7";
             opacity: 0;
         }
+        &:not(:empty)::after {
+            margin-left: 6px;
+            position: absolute;
+        }
         &.o-sort-up {
             cursor: n-resize;
             &::after {