Fix NPE when verify changes without codestyle check

When verification is triggered on stable branches
the codestyle verification is not executed and thus
the resulting object is null.

Change-Id: I091ab8bdac559e944dd8fc85de9d4ba8437ee3cc
diff --git a/jenkins/gerrit-verifier-change.groovy b/jenkins/gerrit-verifier-change.groovy
index f221163..bcb7345 100644
--- a/jenkins/gerrit-verifier-change.groovy
+++ b/jenkins/gerrit-verifier-change.groovy
@@ -321,7 +321,7 @@
 
   def resVerify = buildsWithResults.findAll{ it != codestyleResult }.inject(1) { acc, buildResult -> getVerified(acc, buildResult[1]) }
 
-  def resAll = getVerified(resVerify, codestyleResult[1])
+  def resAll = codestyleResult ? getVerified(resVerify, codestyleResult[1]) : resVerify
 
   gerritLabelVerify(change, sha1, resVerify, Globals.buildsList.findAll { key,build -> key != "codestyle" })