[FIX] web: one2many displayed even if no data.

Before this commit, the one2many widget was not displayed if no data.
That make some form (notebook) empty while the user don't click en edit to see the widget appear.

Eg: stock picking 'Initial Demand' tab

Courtesy of @ged-odoo

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

@@ -1275,6 +1275,9 @@ var FieldOne2Many = FieldX2Many.extend({
             return self.mutex.def;
         });
     },
+    is_false: function() {
+        return false;
+    },
 });
 
 var Many2ManyListView = X2ManyListView.extend({