account: fix empty vector access

NewAccountModelPimpl::slotProfileUpdated() does an access on first
element on a vector without checking if it's not empty.
This results in immediate crash.
Fixed by checking the empty condition and doing nothing in such case.

Change-Id: I49276e7ddc7034ebac24d1c109313751f06b4746
Tuleap: #1779
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>