fix "Without projects" filter

What does this MR do?

Fix issue where "Without projects" filter admin area shows 0 users incorrectly. Before this fix, if any outstanding group or project invitations exist the count show as 0.

Are there points in the code the reviewer needs to double check?

Seems pretty straight forward.

Why was this MR needed?

Bug

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3367