If there is a network failure the return value from latest_versionstring() is
('','') which later causes an exception when comparing versions.
Improve this by checking the return value and failing the test early.
[ YOCTO #12053 ]
(Bitbake rev: 3f034d2172bf64ecc43577b43e0cf032a54b1358)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>