update for fix issue JabRef#6199, replace null with emptyList() to av…

…oid null check.

Co-authored-by: Christoph <siedlerkiller@gmail.com>

src/main/java/org/jabref/logic/importer/fileformat/EndnoteXmlImporter.java

@@ -261,7 +261,7 @@ private Optional<String> getUrl(Record record) {
 
     private Optional<String> getUrlValue(Url url) {
         Optional<List<Object>> urlContent = Optional.ofNullable(url).map(Url::getContent);
-        List<Object> list = urlContent.orElse(null);
+        List<Object> list = urlContent.orElse(Collections.emptyList());
         Optional<String> ret;
         if (list==null || list.size() == 0) {
             return Optional.empty();