LDAP: Fix get_value on undefined value for external_entry

I do not know, in which cases the external_entry is specified by RT and in
which not.
The logs show this error occurred since 01.07.2024

Taken from the documentation:
https://github.com/bestpractical/rt/blob/rt-4.4.6/lib/RT/Authen/ExternalAuth.pm#L261
"The subroutine is called in 2 modes: when called with external_entry
specified, it should return value or list of values, otherwise, it should
return the external field list it depends on, so RT could retrieve them at the
beginning."
"E.g.

    YYY => sub {
        my %args = @_;

        return 'XXX' unless $args{external_entry};

        my @values = grep defined && length, $args{external_entry}->get_value('XXX');
        return @values;
    },"