[FIX] website_slides: Don't set completion_time for encrypt/corrupt PDF

Steps to reproduce:

  - Install eLearning module
  - Go to eLearning, open course Basics of Gardening and edit it
  - Click on `How to Grow and Harvest The Best Strawberries | Basics`
  - Upload an encrypted PDF as Attachement

Issue:

  Traceback is raised :
  utils.PdfReadError("File has not been decrypted")

Cause:

  When trying to calculate `completion_time` to read pages, we try to
  get the number of pages of the pdf but an error will be raised if the
  pdf is encrypted.

Solution:

  If not possible to get number of pages, we dont set/update the
  completion_time.

opw-2612529

closes odoo/odoo#76745

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>