[FrameworkBundle] fixes displaying of deprecation notices.

Author: Hugo Hamon

Resources/config/debug_prod.xml

@@ -16,7 +16,7 @@
             <tag name="monolog.logger" channel="php" />
             <argument /><!-- Exception handler -->
             <argument type="service" id="logger" on-invalid="null" />
-            <argument /><!-- Log levels map for enabled error levels -->
+            <argument>null</argument><!-- Log levels map for enabled error levels -->
             <argument>null</argument>
             <argument>true</argument>
             <argument>null</argument><!-- %templating.helper.code.file_link_format% -->