ci.jpl: Initialize "output" variable

Otherwise it's possible to get error like:

groovy.lang.MissingPropertyException: No such property: output for class: groovy.lang.Binding

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I83f7cfd350d111b2c8cc362904a4694be84ce923
diff --git a/jenkins/ci.jpl b/jenkins/ci.jpl
index 7514c67..1353e8d 100644
--- a/jenkins/ci.jpl
+++ b/jenkins/ci.jpl
@@ -551,6 +551,7 @@
       print("Wait for LAVA results here...")
       try {
         all_jobs = submitJobsToList(results['lava_jobs'])
+        output = ""
         if (all_jobs.size() > 0) {
           dir("tf-m-ci-scripts") {
             withCredentials([usernamePassword(credentialsId: env.LAVA_CREDENTIALS, passwordVariable: 'LAVA_TOKEN', usernameVariable: 'LAVA_USER')]) {