Fix getAllPrefData when the item value is null

Fix getAllPrefData when the item value is null

Author: haofei91

debug-db/src/main/java/com/amitshekhar/server/ClientServer.java

@@ -455,10 +455,10 @@ public Response getAllPrefData(String tag) {
         for (Map.Entry<String, ?> entry : allEntries.entrySet()) {
             List<String> row = new ArrayList<>();
             row.add(entry.getKey());
-            row.add(entry.getValue().toString());
+            row.add(entry.getValue() != null ? entry.getValue().toString() : "");
             response.rows.add(row);
         }
         return response;
     }
 
-}
+}