Print a notice when skipping cache operation due to empty cache key

What does this MR do?

Adds a notice printed when cache operation is skipped because of empty cache key.

Why was this MR needed?

Please read #3112 (closed) for a reference. This change would also help in investigating problems like the one in gitlab-ce#43373.

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #3112 (closed)