[FIX] web: avoid margin on top of list view in mobile

When header was too long, the text was breaking into next line.
We want to avoid this behavior in mobile because it causes
a big margin on the top of the list view.

This issue was probably caused by the upgrade of Bootstrap.

task-1878252

closes odoo/odoo#27927

addons/web/static/src/scss/list_view.scss

@@ -23,6 +23,9 @@
         border-bottom: 1px solid #cacaca;
         > tr > th {
             border-left: 1px solid #dfdfdf;
+            @include media-breakpoint-down(sm) {
+                white-space: nowrap;
+            }
             &:not(.o_column_sortable):hover {
                 cursor: default;
             }