[5.x] Fix broken revision links on unpublished entries

[faltjo]

Fixes: #10331

When clicking on the 'View History' button of an unpublished entry in the CP, the revision history panel loads indefinitely.
The called route cp/collections/{collection}/entries/{entry}/revisions fails with error 500.

Illuminate\Routing\Exceptions\UrlGenerationException
Missing required parameter for [Route: statamic.cp.collections.entries.revisions.show] [URI: cp/collections/{collection}/entries/{entry}/revisions/{revision}] [Missing parameter: revision].

Apparently the working copy of the entry included in the entry's list of revisions has no id, as it is not associated with another revision. But this id is necessary to create a link in the list of revisions.

Using an empty link fixes this problem, as the working copy only exists locally and no revision shall be showed on click anyway.

Note: I was not able to find any tests of the EntryRevisionsController so I adapted a test related to this use-case. Feel free to shift it to the correct location.

See also: #10057

[duncanmcclean]

Thanks for the pull request!

I've tweaked the if statement and added a test for the "revisions index" endpoint to make sure it works as expected. 👍