[FIX] web: more reliably avoid downloads on new file

51955505 mitigated an issue of being
able to try and download files which don't exist yet, make the fix
more reliable by clearing out the field completely and hiding the
content if the (readonly) field has no value *or the record is not
saved yet*.

Also clean up the code:

* an old-style forward port created a duplicate fixprovement
  (a8d01cbf) which seems less correct
  as it applies conditionally
* and the code is branchier than necessary, we can make it simpler by
  judiciously leveraging jquery's API

closes odoo/odoo#77603

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>