SONAR suggestion - toString() should not return null

Change-Id: I9b31515b24052d60895ef2151609ea734ef18c3b
diff --git a/utils/misc/src/main/java/org/onlab/packet/DHCPPacketType.java b/utils/misc/src/main/java/org/onlab/packet/DHCPPacketType.java
index 2dad848..c88b907 100644
--- a/utils/misc/src/main/java/org/onlab/packet/DHCPPacketType.java
+++ b/utils/misc/src/main/java/org/onlab/packet/DHCPPacketType.java
@@ -80,7 +80,7 @@
             break;
         }
 
-        return null;
+        return "";
     }
 
     public static DHCPPacketType getType(final int value) {