[IMP] various: Hide default user image for unassigned records

Impacted modules: account_analytic_default, crm, event,
hr_recruitment, maintenance, mass_mailing, project,
purchase_requisition, stock_picking_batch, website_slides

The default image displayed when the record is unassigned is
confusing, so we are removing it.

FP request
Task 2234524

closes odoo/odoo#49333

Related: odoo/enterprise#9834
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>