Clear `displayPrefix` in `makeEmptySourceAccessor` (backport #13296)

[mergify[bot]]

Motivation

Judging by the comment for makeEmptySourceAccessor the prefix has to be empty:

Return a source accessor that contains only an empty root directory.

Context

Fixes #13295.

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This is an automatic backport of pull request #13296 done by [Mergify](https://mergify.com).

[mergify[bot]]

This pull request has been removed from the queue for the following reason: checks failed.

The merge conditions cannot be satisfied due to failing checks:

• ❌ vm_tests

You may have to fix your CI before adding the pull request to the queue again. If you update this pull request, to fix the CI, it will automatically be requeued once the queue conditions match again. If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.

[roberth]

Can't reproduce the vm_tests failure. Could be detsys action adjacent, @edolstra?