Correctly check for empty value, instead of using empty().

Fixes #670

CHANGELOG.md

@@ -13,6 +13,7 @@ Exclamation symbols (:exclamation:) note something of importance e.g. breaking c
 ### Fixed
 - SQL update script for version 0.44.1-0.45.0.
 - Issues found by Scrutinizer (Type hints and return values).
+- Check inline keyboard button parameter value correctly. 
 ### Security
 
 ## [0.49.0] - 2017-09-17

src/Entities/InlineKeyboardButton.php

@@ -64,7 +64,7 @@ class InlineKeyboardButton extends KeyboardButton
         $num_params = 0;
 
         foreach (['url', 'callback_data', 'switch_inline_query', 'switch_inline_query_current_chat', 'pay'] as $param) {
-            if (!empty($this->getProperty($param))) {
+            if ($this->getProperty($param, '') !== '') {
                 $num_params++;
             }
         }