Empty choice of Form Template yields in error during rendering

[tschale]

Per default, Form Template is an empty choice. This used to work, but since I updated to python 3.4, Django 1.11.11 and django-cms 3.5.1 on Ubuntu 14.04, this produces an error during template rendering. On FormPlugin.get_render_template, the instance.form_template is an empty string, which produces the error, as select_template only excepts a TemplateDoesNotExist error and not the necessary IsADirectoryError.