[FIX] website_slides: hide the "Go to website" button if the slide is not saved

Steps to reproduce the bug:
- Go to Elearning > choose any course or create a new one
- Click on “add content” :
    - Add a title to slide
    - Click on "Go to Website"

Problem:
An error is triggered because we call the "create" method to save the slide, which tries to access the “channel_id”, the course ID in which the slide will be added. However, the field is not set in values.
The "create" or "write" function in "slide_channel" must first be called to save the course, which will then call the create function in "slide_slide"

Solution :
Hide the "Go to website" button as long as the course is not created and therefore has no id

opw-2526541

closes odoo/odoo#71739

Signed-off-by: Djamel Touati <DjamelTouati@users.noreply.github.com>