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).

ci-scripts/Jenkinsfile-trig-nsa

@@ -20,6 +20,9 @@
  *      contact@openairinterface.org
  */
 
+// necessary for reading JSON
+import groovy.json.JsonSlurper
+
 // Template Jenkins Declarative Pipeline script to run Test w/ RF HW
 
 // Location of the python executor node shall be in the same subnet as the others servers
@@ -38,11 +41,11 @@ pipeline {
         stage ("Launcher") {
             steps {
                 script {
-                    //retrieve MR that are opened nd with tag NSA             
-                    MR_LIST= sh returnStdout: true, script: 'curl --silent "https://gitlab.eurecom.fr/api/v4/projects/oai%2Fopenairinterface5g/merge_requests?state=opened&per_page=100&labels=NSA" | jq ".[].iid" || true '
-                    echo "List of selected MR:\n${MR_LIST}" 
-                    def MR_ARRAY = MR_LIST.split('\n') 
-                    //for every selected MR, retrieve the branch name and the latest commit              
+                    // retrieve MR that are opened nd with tag NSA
+                    MR_LIST = sh returnStdout: true, script: 'curl --silent "https://gitlab.eurecom.fr/api/v4/projects/oai%2Fopenairinterface5g/merge_requests?state=opened&per_page=100&labels=NSA" | jq -cj "[.[].iid]"'
+                    echo "List of selected MRs: ${MR_LIST.trim()}"
+                    def MR_ARRAY = new JsonSlurper().parseText(MR_LIST.trim())
+                    // for every selected MR, retrieve the branch name and the latest commit
                     for (MR in MR_ARRAY) {
                         SRC_BRANCH=sh returnStdout: true, script: """curl --silent "https://gitlab.eurecom.fr/api/v4/projects/oai%2Fopenairinterface5g/merge_requests/${MR}" | jq ".source_branch" || true """
                         SRC_BRANCH=SRC_BRANCH.trim()