Fix the service definitions when the logger is not set in listener

The empty tag is parsed to an empty string, not to null. And this is not
a valid value for the service

Author: stof

Resources/config/mongodb.xml

@@ -26,7 +26,7 @@
             <argument type="collection" /> <!-- events -->
             <argument type="service" id="fos_elastica.indexable" />
             <argument type="collection" /> <!-- configuration -->
-            <argument /> <!-- logger -->
+            <argument>null</argument> <!-- logger -->
         </service>
 
         <service id="fos_elastica.elastica_to_model_transformer.prototype.mongodb" class="%fos_elastica.elastica_to_model_transformer.prototype.mongodb.class%" public="false" abstract="true">

Resources/config/orm.xml

@@ -25,7 +25,7 @@
             <argument type="collection" /> <!-- events -->
             <argument type="service" id="fos_elastica.indexable" />
             <argument type="collection" /> <!-- configuration -->
-            <argument on-invalid="ignore" /> <!-- logger -->
+            <argument>null</argument> <!-- logger -->
         </service>
 
         <service id="fos_elastica.elastica_to_model_transformer.prototype.orm" class="%fos_elastica.elastica_to_model_transformer.prototype.orm.class%" public="false" abstract="true">