scripts: detect EOF on read in a usual way in systemd-autofs-restart.sh

When read bultin detects EOF, it sets empty string as value of specified
variable, and returns non-zero exit code. So we can rely just on exit
code, and not check variable value. This is standard approach.
https://jira.sw.ru/browse/PSBM-44207



Signed-off-by: Eugene Kolomeetz <klm@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>