findbugs: toString return null

its a bad practice and may cause to runtime exception when returning
null in toString()

findbugs reason
NP: toString method may return null (NP_TOSTRING_COULD_RETURN_NULL)

Change-Id: Ib7385775b938054a5fd713c458706271b20573ad
Signed-off-by: Shahar Havivi <shavivi@redhat.com>