Don't refocus a workspace cleaned up by `workspace_show` during rename

When moving a workspace to the current output by way of a rename, if the
current workspace is empty, it will be removed by `workspace_show`.
Attempting to restore focus to this removed workspace causes a crash.
Follow the pattern in workspace.c:996 to only restore the original focus if the
original workspace still exists.

Add a test to ensure that the renamed workspace moves to its appropriate
output and that a crash does not occur.

Fixes #3228