Remove a redundant condition and an empty string

Remove "NULL.equals(object)" on line 2756 of JSONObject.java since line 2752 has already tested it.
Remove the empty string on line 249 of JSONPointer.java.

src/main/java/org/json/JSONPointer.java

@@ -246,7 +246,7 @@ public class JSONPointer {
      */
     @Override
     public String toString() {
-        StringBuilder rval = new StringBuilder("");
+        StringBuilder rval = new StringBuilder();
         for (String token: this.refTokens) {
             rval.append('/').append(escape(token));
         }