support "" equals to None check in Testcase.is_compatible
support "" equals to None check in Testcase.is_compatible

when check ci_installer/ci_scenario is None, we also check if ci_installer/ci_scenario is ""

JIRA: FUNCTEST-404

Change-Id: Ib8b7b9a98f6eb064d2b3648e5d1334990c69cb2f
Signed-off-by: SerenaFeng <382e42c9362c05401b97e548e66ecc0826ab4b31@zte.com.cn>

opnfv/functest,mywulin/functest,opnfv/functest,mywulin/functest