Advanced Search: fix project count_only queries

What does this MR do?

When you perform a search for any type other than projects at the group level you end up searching the projects tab will show an incorrect count because it is matching all projects in the group without using the search term you entered. Switching to the tab you'll see that it's not as many projects as it initially shows.

This MR fixes the problem with ProjectClassProxy, which has been introduced in !51971 (merged)

#321620 (closed)

How to reproduce

1. Load an issues search for definitelynomatchingprojects
2. Notice the the project tab count is over 1k results
3. Click on the project tab
4. See there are actually no matching projects at all

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #321620 (closed)