[Deprecation] Null as array offset is deprecated

[KDederichs]

Describe the bug
After updating to PHP 8.5 I got:

Deprecated: Using null as an array offset is deprecated, use an empty string instead in /app/vendor/easycorp/easyadmin-bundle/src/Router/AdminRouteGenerator.php on line 938
PHP Deprecated:  Using null as an array offset is deprecated, use an empty string instead in /app/vendor/easycorp/easyadmin-bundle/src/Router/AdminRouteGenerator.php on line 938

To Reproduce
Use PHP 8.5

(OPTIONAL) Additional context
I'll see if I can fix it over the weekend, if not or if someone is faster, be my guest :)

[javiereguiluz]

Closed by #7279.