Fix EZP-22738: Fatal error in Legacy\View\Provider\Block

https://jira.ez.no/browse/EZP-22738

Sometimes in `Legacy\View\Provider\Block`, legacy kernel fails to return an `eZObjectForwarder` for `block_view_gui`. This happens most likely when there is a cache issue (e.g. context change when developing).
It results to a fatal error:

```
PHP Notice:  Undefined index: block_view_gui in 
/Users/lolautruche/workspace/ezsystems/ezpublish-legacy/lib/eztemplate/classes/eztemplate.php 
on line 660

PHP Fatal error:  Call to a member function process() on a non-object in 
/Users/lolautruche/workspace/ezsystems/ezpublish-kernel/
eZ/Publish/Core/MVC/Legacy/View/Provider/Block.php on line 59
```

Potentially also occurs with `Legacy\View\Provider\Content`.