[FIX] web: FormViewDialog: multi select option

Before this rev., the multi_select option wasn't enabled even by setting
disable_multiple_selection to false in the options, when the res_id key
wasn't defined in options (null != undefined). It was the case for every
X2Many views, except for the Many2ManyKanban view as we explicitely set
the disable_multiple_selection to false AND res_id to null. However, all
X2Many views should enable this option.

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

@@ -841,8 +841,8 @@ var FormViewDialog = ViewDialog.extend({
     init: function(parent, options) {
         var self = this;
 
-        var multi_select = options.res_id === null && ! options.disable_multiple_selection;
-        var readonly = options.res_id !== null && options.readonly;
+        var multi_select = !_.isNumber(options.res_id) && !options.disable_multiple_selection;
+        var readonly = _.isNumber(options.res_id) && options.readonly;
 
         if(!options || !options.buttons) {
             options = options || {};

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

@@ -1654,13 +1654,11 @@ odoo.define('web_kanban.Many2ManyKanbanView', function (require) {
             var self = this;
             new common.FormViewDialog(this, {
                 res_model: self.x2m.field.relation,
-                res_id: null,
                 domain: self.x2m.build_domain(),
                 context: self.x2m.build_context(),
                 title: _t("Create: ") + self.x2m.string,
                 initial_view: "form",
                 alternative_form_view: self.x2m.field.views ? self.x2m.field.views.form : undefined,
-                disable_multiple_selection: false,
                 create_function: function(data, options) {
                     return self.x2m.data_create(data, options);
                 },