[FIX] website_slides: inaccurate completion time

How to reproduce the bug
=========================.
1. Open e-Learning.
2. Create a section.
3. Add a content in the created section and change the default duration .
-> New duration is not set and it is taking the default duration only

Technical
=========
https://github.com/odoo/odoo/commit/7b7fdf8f2771840797e1d416a8275ba3666ab542


This commit introduce the feature to set default time for the content in the
backend if user put the Duration field empty for that content but because of
the extra 'not' in the condition it was always getting true so every time
default value is stored and if completion_time is not there then it is set
to 0.

After this Commit
==================
Now whatever duration user will put, it will be set and if completion_time is
not given then it will give us the default time.

Task-3340462

closes odoo/odoo#135334

X-original-commit: 1c470b65bca48c2ac660fda919be7b8fc6e3990f
Signed-off-by: Stéphane Debauche (std) <std@odoo.com>