[FIX] account: don't recompute name when no date set

After commit 9bf54f09, we assumed that
the date field of a move was never empty. On the form view, it can be
(temporarily) empty though (before save), causing an error when trying
to read it.

We now don't try to recompute the name when the date is empty, thus
preventing the date field to be read in that method.

task-3326834

closes odoo/odoo#122260

Signed-off-by: William André (wan) <wan@odoo.com>