Fix crash in preBackup {"tempDb": ""}

:Release Notes:
Fix range_error when referring to the last character of empty string.

:Detailed Notes:

:Testing Performed:
luna-send -n 1 luna://com.palm.systemservice/backup/preBackup '{"tempDb": ""}'

:QA Notes:

:Issues Addressed:
[GF-57396] LunaSysService crashes at location /lib/libc.so.6(gsignal+0x3c)

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I3c143b17697200a075e1f750eb7b03ca5218af35
Reviewed-on: https://g2g.palm.com/4324
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>