If a schema 1 manifest is uploaded with the `disablesignaturestore` option set
to true, then no signatures will exist. Handle this case.
If a schema 1 manifest is pushed, deleted, garbage collected and pushed again, the
repository will contain signature links from the first version, but the blobs will
not exist. Disable the signature store in the garbage-collect command so
signatures are not fetched.
Signed-off-by: Richard Scothern <richard.scothern@docker.com>
31ece3d3b6