Move UnequalStashexception inside if

ci/Jenkinsfile

@@ -41,7 +41,6 @@ def abortPreviousRunUnlessMaster() {
  */
 def runDockerWithPostgresSidecar(String command,  List<String> stashsrc = [], List<String> stashdir = []) {
     return {
-        if (stashsrc.size() != stashdir.size()) { throw new UnequalStashException() };
         node('docker') {
             node_info()
             checkout scm
@@ -52,8 +51,13 @@ def runDockerWithPostgresSidecar(String command,  List<String> stashsrc = [], Li
                     wait_for_db()
                     sh './ci/build-scripts/build-schema.sh'
                     sh "cd backend && ${command}"
-                    for (int i = 0; i < stashsrc.size(); i++) {
-                        stash includes: stashsrc[i], name: stashdir[i]
+                    if (stashsrc && stashdir) {
+                        if (stashsrc.size() != stashdir.size()) {
+                            throw new UnequalStashException()
+                        }
+                        for (int i = 0; i < stashsrc.size(); i++) {
+                            stash includes: stashsrc[i], name: stashdir[i]
+                        }
                     }
                 }
             }
@@ -273,6 +277,6 @@ class PackageLockJsonModifiedException extends Exception {
 
 class UnequalStashException extends Exception {
     UnequalStashException() {
-        super("You either gave too many stash sources or targets.")
+        super("The stashed files count in stashsrc and stashdir lists must be equal.")
     }
 }