LUCENE-2438: We decided that bytes should never be null, so i changed the recently added external serialization methods to conform to this.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@957926 13f79535-47bb-0310-9956-ffa450edef68

lucene/src/java/org/apache/lucene/util/BytesRef.java

@@ -294,7 +294,7 @@ public final class BytesRef implements Comparable<BytesRef>, Externalizable {
       bytes = new byte[length];
       in.read(bytes, 0, length);
     } else {
-      bytes = null;
+      bytes = EMPTY_BYTES;
     }
   }
 }