guard against NPE, bugzilla 39648

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@409022 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/types/resources/Resources.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2005 The Apache Software Foundation
+ * Copyright 2005-2006 The Apache Software Foundation
  *
  *  Licensed under the Apache License, Version 2.0 (the "License");
  *  you may not use this file except in compliance with the License.
@@ -207,7 +207,7 @@ public class Resources extends DataType implements ResourceCollection {
         if (isReference()) {
             return getCheckedRef().toString();
         }
-        if (coll.size() == 0) {
+        if (coll == null || coll.isEmpty()) {
             return "";
         }
         StringBuffer sb = new StringBuffer();