Automatic merge from submit-queue (batch tested with PRs 39311, 39226, 39445)

Make kubectl proxy accept empty path

**What this PR does / why we need it**:
The kubectl proxy previously returned 403 Forbidden: Unauthorized when receiving a request from e.g. "curl localhost:8001" or "curl localhost:8001/".  The previous DefaultPathAcceptRE regex was wrong as it assumed the path in this case would be "/" (but it is actually "").

After someone runs kubectl proxy and tries accessing it with curl, they will probably just try hitting localhost:8001  (which returns an "Unauthorized" response) instead of say localhost:8001/api (which returns a proper response from the API server). Also, whoever previously modified the DefaultPathAcceptRE regex was obviously expecting the regex to accept requests for localhost:8001/

```release-note
fix issue with kubectl proxy so that it will proxy an empty path - e.g. http://localhost:8001
```