Update CmfBlockHelper.php to work with CmfBlockExtension and sonata_bloc...

...k_render

The CmfBlockExtension extends the Sonata\BlockBundle\Twig\Extension\BlockExtension which defines `sonata_block_render` in its (extended) getFunctions() method.  The `sonata_block_render` simple Twig function will call the CmfBlockHelper render() method, so it should be more like the Sonata\BlockBundle\Templating\Helper\BlockHelper render() method.

An exception has been thrown during the rendering of a template ("Notice: Undefined offset: 1 in /vendor/symfony-cmf/block-bundle/Symfony/Cmf/Bundle/BlockBundle/Templating/Helper/CmfBlockHelper.php line 64")

The template code that triggered this error was similar to:
```jinja
{{ sonata_block_render({ 'name': '/blocks/Test/case' }, { 'template': 'ExampleBundle:Test:Block/block_simple.html.twig' }) }}
```

ping @dbu