Use a minimal size of 2 for the select tag of MultiSelection widgets

Browsers based on Chromium renders the select as a "normal" select tag when
size=1. Such that the mousedown event is triggered twice which clear the
selection.

Closes #14529

sao/src/view/form.js

@@ -4910,7 +4910,8 @@ function eval_pyson(value){
             var record = this.record;
             var field = this.field;
             this.update_selection(record, field, () => {
-                this.select.prop('size', this.select.children().length);
+                this.select.prop(
+                    'size', Math.max(2, this.select.children().length));
                 if (!field) {
                     return;
                 }