utils/genrandconfig: add refpolicy custom git handling

Add a custom case to make sure that a random configuration with an empty
git location for refpolicy doesn't fail. It reverts to
BR2_PACKAGE_REFPOLICY_UPSTREAM_VERSION in that case.

Fixes:
 - http://autobuild.buildroot.org/results/98da07a1f9b88e2e8a028a55c8cf76c6ebc28304



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>