Fix EZP-23332: Templating Legacy Engine throws warning if TemplateRefere...

...nce is passed as param to supports method

Fixes https://jira.ez.no/browse/EZP-23332

Found this issue when trying to add a translation webinterface to my project
http://jmsyst.com/bundles/JMSTranslationBundle/master/webinterface
Following all the steps there and pointing your browser to [your_dev]/_trans, we have a warning saying

```
ContextErrorException: Warning: Illegal offset type in isset or empty in [...]/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Legacy/Templating/LegacyEngine.php line 136
```



@andrerom @lolautruche should we apply @patrickallaert suggestion here? Makes sense to me. i can take if you want.