When [[Special:GlobalUserRights]] is called without user parameter, the special page code always tries to initialize an user calling SpecialGlobalGroupMembership::fetchUser.
As we don't have an user that throws the following exception:
MapCacheLRU::has called with invalid key. Must be string or integer.
The expected behavior is a form to be able to offer an username.
This can be checked at https://meta.wikimedia.org/wiki/Special:GlobalUserRights.