[FIX] web: prevent missing 'data' argument error while uploading empty file

DataURL returns 'data:' in Chrome for empty files which is not valid dataURLas
it expects 'data:<mimetype>;base64' . This DataURL method is used to obtain
base64 representation of the file data,. That's why the 'data' argument is
missing and error is being generated.

Steps to reproduce:
1) Open Notes and create a new Note in Chrome.
2) Add a new attachment using /image command.
3) Add an empty file.

By applying this,it will return proper format for empty file.

sentry - 3749971166

closes odoo/odoo#112416

Signed-off-by: Samuel Degueldre <sad@odoo.com>