Build fix for superproject fetch jobs
!29 (merged) didn't account for the possibility that the list of submodules where the default branch and the CrossCompilationSupport branch had diverged was empty. Also apply the same fix for the case where the list of submodules on the CrossCompilationSupport branch is empty, as one day that will (hopefully) be the case also.
Should fix BuildHost, iMx6 and OMAP5 pipelines.