[refactor] allow null value in item separator again

This is just to safeguard against possible uses of
FunSerialize.normalize we don't know of.

Author: line-o

exist-core/src/main/java/org/exist/xquery/functions/fn/FunSerialize.java

@@ -186,6 +186,7 @@ public static Sequence normalize(final Expression callingExpr, final XQueryConte
         try {
             final MemTreeBuilder builder = context.getDocumentBuilder();
             final DocumentBuilderReceiver receiver = new DocumentBuilderReceiver(callingExpr, builder, true);
+            final String safeItemSeparator = itemSeparator == null ? "" : itemSeparator;
             for (final SequenceIterator i = step2.iterate(); i.hasNext(); ) {
                 final Item next = i.nextItem();
                 if (Type.subTypeOf(next.getType(), Type.NODE)) {
@@ -196,7 +197,7 @@ public static Sequence normalize(final Expression callingExpr, final XQueryConte
                 }
                 // add itemSeparator if there is a next item
                 if (i.hasNext()) {
-                    receiver.characters(itemSeparator);
+                    receiver.characters(safeItemSeparator);
                 }
             }
             return (DocumentImpl)receiver.getDocument();

exist-core/src/main/java/org/exist/xquery/functions/util/Eval.java

@@ -404,7 +404,8 @@ private Sequence doEval(final XQueryContext evalContext, final Sequence contextS
 
                         final Sequence seq;
                         if (xqSerializer.normalize()) {
-                            seq = FunSerialize.normalize(this, context, result, "");
+                            // TODO(JL): should this not be changed to DEFAULT_ITEM_SEPARATOR
+                            seq = FunSerialize.normalize(this, context, result, null);
                         } else {
                             seq = result;
                         }