runner/settings: Be consistent with empty blacklist

If provided an empty blacklist let's fail instead of returning a value
of uninitialized variable.

Cc: Oleg Vasilev <oleg.vasilev@intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>