jenkins/update2head: skip push if nothing available to commit

BugzID_ 73789
Signed-off-by: Marc Mattmueller <marc.mattmueller@netmodule.com>

Jenkinsfile_update2head

@@ -107,5 +107,10 @@ def commitChanges(isNmUpdate, isCommunityUpdate) {
   if(isCommunityUpdate) { updatedLayers += "community" }
   sh(script: "git checkout ${env.BRANCH_NAME}")
   sh(script: "git add -u")
+  stagedData = sh(returnStdout: true, script: "git diff --cached").trim()
+  if("${stagedData}" == "") {
+    println "everything up to date, nothing to commit"
+    return
+  }
   sh(script: "git commit -m \"submodules: updated ${updatedLayers} hashes, triggered by ${env.TRIGGERED_USER}\" && git push")
 }