setSpec null pointer because of wrong if clause

src/main/java/org/gesis/dda/publishing/domain/impl/XsltTransformerOaiPmhBundlesStreamSource.java

@@ -135,7 +135,7 @@ public class XsltTransformerOaiPmhBundlesStreamSource implements BundlesStreamSo
                 stream().
         flatMap(mp -> {
             LOG.debug("filling list identifier stream with mp {}, from {}, until {}, setSpec {}", mp, from, until, setSpec);
-            if (setSpec.isEmpty() || setSpec == null) {
+            if (setSpec == null|| setSpec.trim().isEmpty()) {
                 return client.listIdentifiersStream(mp, from, until, null);
             }
             return client.listIdentifiersStream(mp, from, until, setSpec);