ComboGrid: fix on-load validation for blank values

ComboGrid: fix on-load validation for blank values

Commit f32aa3df74 fixed marking multi-select fields with where the store
did not contain a valid value after loading.

However, it introduced a bug for single-select fields where the value
(before the store-load) was explicitly set to be empty (when that should
be invalid because of allowBlank === false).

Fix the logic to correctly detect all scenarios (with def being the
value selected before the store loaded, i.e. undefined or an empty
array):

  !allowBlank &&
    ( def is an array but empty || def is not an array and falsy )

Also use correct error message (localized by ExtJS itself).

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>