EZP-22290: Avoid PHP notice because of uninitialized variable

In `TemplateDebugInfo::getLegacyTemplatesList` a PHP Notice is generated when the usage statistics can not be retrieved thus leaving `$templateStats` uninitialized.

```php
try
{
    $templateStats = $legacyKernel()->runCallback(
        function ()
        {
            return eZTemplate::templatesUsageStatistics();
        }
    );
}
catch ( RuntimeException $e )
{
    // Ignore the exception thrown by legacy kernel as this would break debug toolbar (and thus debug info display).
    // Furthermore, some legacy kernel handlers don't support runCallback (e.g. ezpKernelTreeMenu)
}

foreach ( $templateStats as $tplInfo )
{
    // ...
}
```

This PR adds the variable declaration in the `catch` block.

https://jira.ez.no/browse/EZP-22290

Cheers
:octocat: Jérôme