Modified find project method to return nil if project not found

What does this MR do?

Files that belong to nonexistent projects get silently ignored.

Why was this MR needed?

To prevent the nonexistent files from showing up in search.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5300 (closed) - 404 error from the search controller when the elasticsearch index is out of date