Support dialog without action button

Since 5eb06085 (erp5_web_renderjs_ui: do not display submit and cancel if action is empty in form_dialog., 2020-06-10) ERP5JS does not display submit button, this continues in this direction to:

• Apply the same possibility in xhtml_style to not show a submit button when there's no action. In xhtml_style we show the cancel button using the same rules as everywhere else.
• Fix error in ERP5JS for dialogs with only an update action but no action.