[cog] Handle "No modified repo paths" output from git citc.

The `_affected_cog_repos` property now checks for the specific output "No modified repo paths" from `git citc api.get-modified-repos` and returns an empty set in that case. The test fake was updated to match this behavior.

Change-Id: I137d5e282a39dad1d50817867108598c65d6d2fe
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1646855
Commit-Queue: Jiaming Li <lijiaming@google.com>
Reviewed-by: Chase Latta <chaselatta@google.com>
Fuchsia-Auto-Submit: Jiaming Li <lijiaming@google.com>