perms: update test to cover no-update case

Created by: unknwon

Alter our test to cover the case of the following "if" (i.e. skip when no rows need to be updated):

https://github.com/sourcegraph/sourcegraph/blob/30670310f1b0a8a30fc11ac3079043382eda2fd3/enterprise/internal/db/perms_store.go#L660-L666

The test still does what it was intended to do, and now covers one more case.

Fixes #14761