Remote property accessor on branch branch should not throw an exception

When a branch is under the refs/remotes/ namespace, but a specific remote cannot be resolved,
then the Remote property accessor should return null instead of throwing an exception.
Yeah, that's what I would expect from such an API. "not found" leads me to believe that I gave it something which doesn't exist, when the case is that I gave it something which exists, but is of the wrong type.