[FIX] web_kanban: in non-grouped kanban, on switch to form, display first record

in view_form.js, in do_show method, if the dataset index is not set, the on_button_new method is triggered, opening a new record
In regular kanbans (non-grouped kanban), the dataset index wasn't set, and, therefore, when switching to the view mode, it opened a form for a new record instead of opening the first record of the list, like this is the case for the view list and the grouped kanban