[FIX] website: allow to remove shadow on all cards

Steps to reproduce:

  - Install `website_hr_recruiment` module
  - Go to `your_website.com/jobs`
  - Activate Edit mode and add a "3 Columns" block
  - Select one of the columns and set the shadow to `None`

Issue:

  Shadow is not removed, and by default the "outset" mode is selected.

Cause:

  When we check the state of the shadow with `css('box-shadow')`, it
  will always be set because a custom css 'box-shadow' is set by the
  module website_hr_recruitment on card element, and therefore the value
  will always be either 'inset' or 'outset'.

Solution:

  If widget value is set to '', replace `box-shadow` style to 'none'
  (only if needed) so it will override the style from css file.

opw-2701512

Part-of: odoo/odoo#81038
Co-authored-by: qsm-odoo <qsm@odoo.com>