EZP-22982: Allow $phpSelf to be identical to $index

With display_errors set to `E_ALL` and nginx, the content tree menu isn't shown in the backend (only the "Internal error" message).

Calling the requested url directly (/content/treemenu/2/.... reveals a JSON encoded PHP notice:
```
Warning: strpos(): Empty needle in /var/www/ezpublish/ezpublish_legacy/lib/ezutils/classes/ezsys.php line 1231"
```

This is a very simple patch - I first worked with a regular expression but didn't want to overcomplicate the code as it is more readable like this.

Cheers
:octocat: Jérôme