[PSR] Fixed library source

- Using `php-cs-fixer` tool.

lib/PayPal/Validation/ArgumentValidator.php

@@ -23,7 +23,7 @@ class ArgumentValidator
         if ($argument === null) {
             // Error if Object Null
             throw new \InvalidArgumentException("$argumentName cannot be null");
-        } else if (gettype($argument) == 'string' && trim($argument) == ''){
+        } elseif (gettype($argument) == 'string' && trim($argument) == '') {
             // Error if String Empty
             throw new \InvalidArgumentException("$argumentName string cannot be empty");
         }