[FIX] website: prevent crash when missing field dependency in forms

Steps to reproduce:
- Add a form.
- Setup a conditional visibility on one field, using the "contains" or
  "does not contain" operator.
- Save.
- Open the HTML editor and voluntarily break the form by changing the
  "data-visibility-dependency" attribute to a random text (non existing
  field used as a dependency).
- Save => Crash on page load.

One customer apparently reached that case of receiving a `null` value
(actually possible when retrieving a form data which is an unchecked
checkbox for example) but combined with an operator which really
requires the received value to be a string... but not sure how this is
possible. In any case, it should not make the form crash: it's just a
problematic *visibility* dependency, it should not prevent using the
form. For now, let's prevent this case to crash and add an error in the
console so we may investigate the issue if it ever shows up in a test.

opw-3243345

closes odoo/odoo#125040

Signed-off-by: Guillaume Dieleman (gdi) <gdi@odoo.com>