[FIX] core: name_get() should return "" instead of False

Because of 43474916, the method
name_get() can now return records with False as label, and this makes
code crash when it expects a string.  Make sure that name_get() always
return strings as labels, while keeping False as empty value for the
field 'display_name'.

closes odoo/odoo#86688

X-original-commit: ff37a2fef89df292b52b1501c28fcf841e1628f5
Signed-off-by: Raphael Collet <rco@odoo.com>