Fix incorrect path/filepath use

What does this MR do?

Fixes a couple of instances here path and filepath were used incorrectly.

This also appears to fix a bug where a cache key can effectively be empty if path traversal is used.

Why was this MR needed?

Fixing the clear mistakes will hopefully help when we tackle the less obvious instances as part of #4524 (closed).

What's the best way to test this MR?

The only logic change can be tested with running the TestGenerateObjectName test.

What are the relevant issue numbers?

Closes #26636 (closed)