Fix NPE in ConcurrentLinkedHashMap

Author: johnthuss

cayenne/src/main/java/org/apache/cayenne/util/concurrentlinkedhashmap/ConcurrentLinkedHashMap.java

@@ -788,6 +788,7 @@ else if (onlyIfAbsent) {
 
     @Override
     public V remove(Object key) {
+    	if (key == null) return null; // this class does allow null to be used as a key or value (returning here prevents an NPE).
         final Node node = data.remove(key);
         if (node == null) {
             return null;