ci: Fix trigger rules

The META_CASSINI_PROJECT and META_CASSINI_TARGET_BRANCH variables are
checked to see which ones are empty. However, since these are not
defined variables, they also need a null check.

CI_MERGE_REQUEST_IID only gets a unique value in the merge
request.

Signed-off-by: Ali Can Ozaslan <ali.oezaslan@arm.com>