Ensure backwards compatible behavior with PHP5

lib/PayPal/Validation/JsonValidator.php

@@ -21,6 +21,9 @@ class JsonValidator
     {
         @json_decode($string);
         if (json_last_error() != JSON_ERROR_NONE) {
+            if ($string === '' || $string === null) {
+                return true;
+            }
             if ($silent == false) {
                 //Throw an Exception for string or array
                 throw new \InvalidArgumentException("Invalid JSON String");