Spike for empty group membership and global excludes

[zackcl]

Exclude users that have no group information from all experiments in a context that has any groups globally excluded.

If the group exclude list for an app context is not empty, the users who have no group information should be excluded from all experiments and feature flags that belong to the app context.

Reference:
25729 users on Production.
group is null, not aliases, since 01-01-25

[amurphy-cl]

#2502