[FIX] web: is_false() for reference field widget. (#9335)

In the widget, the value of reference field is a pair `[val1, val2]`.
When the field is blank, its value is `[false,false]`, thus checking `[false,false] === false`
never succeeds, and this breaks the check for required fields.

addons/web/static/src/js/views/form_widgets.js

@@ -1152,6 +1152,9 @@ var FieldReference = common.AbstractField.extend(common.ReinitializeFieldMixin,
         this.m2o.do_toggle(!!this.get('value')[0]);
         this.reference_ready = true;
     },
+    is_false: function() {
+        return this.get('value')[0] == false || this.get('value')[1] == false;
+    },
 });
 
 var FieldBinary = common.AbstractField.extend(common.ReinitializeFieldMixin, {