web_editor: traceback on saving the product

before this commit:
when creating a product from frontend and editing the product and
delete image, it doesn't get any url thus generating the traceback.

after this commit: if url is not defined then return None from
ir.qweb.field.image field converter

task-2063293

closes odoo/odoo#40466

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>