[FIX] mail: display follower subtypes order by sequence

Before this fix, follower subtypes were displayed by order of id,
whereas in DiscussController.read_subscription_data(),
it was ordered by :
\- parent_mode
\- res_mode
\- internal
\- sequence

See : https://github.com/odoo/odoo/blob/fc115ebdf7c3783f9eaabba71eccce23c676d957/addons/mail/controllers/discuss.py#L471-472
In this commit, we just reproduce the same order.

Also, for `parentModel` and `resModel`, we want the record
with their values unset to come first.
We added 'undefined-first',
as this sort order might be needed for other models.

closes odoo/odoo#93315

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/model/model_field_relation_set.js

@@ -77,6 +77,15 @@ export class RelationSet {
                                     }
                                     break;
                                 }
+                                case 'falsy-first': {
+                                    if (!valA) {
+                                        return -1;
+                                    }
+                                    if (!valB) {
+                                        return 1;
+                                    }
+                                    break;
+                                }
                                 case 'case-insensitive-asc': {
                                     if (typeof valA !== 'string' || typeof valB !== 'string') {
                                         break;

addons/mail/static/src/models/follower_subtype.js

@@ -57,6 +57,8 @@ registerModel({
         parentModel: attr(),
         // AKU FIXME: use relation instead
         resModel: attr(),
-        sequence: attr(),
+        sequence: attr({
+            default: 1,
+        }),
     },
 });

addons/mail/static/src/models/follower_subtype_list.js

@@ -49,6 +49,12 @@ registerModel({
          */
         _sortFollowerSubtypeViews() {
             return [
+                ['falsy-first', 'subtype.parentModel'],
+                ['case-insensitive-asc', 'subtype.parentModel'],
+                ['falsy-first', 'subtype.resModel'],
+                ['case-insensitive-asc', 'subtype.resModel'],
+                ['smaller-first', 'subtype.isInternal'],
+                ['smaller-first', 'subtype.sequence'],
                 ['smaller-first', 'subtype.id'],
             ];
         },