CI: do not trigger tests if nothing is tagged NSA

The previous version would trigger sub-pipelines (LTE-2x2, ...) if no MR had the NSA tag (without a commit, etc). Of course, this would make all pipelines subsequently fail (as not enough arguments, such as the commit, were given). In this commit, we read the output of jq from JSON, which will result in an empty list if no MR is tagged with the NSA label (and the output in the console will be nicer as well).