Modified snapshot to return a non-nil cache even if the snapshot is nil.
Modified snapshot to return a non-nil cache even if the snapshot is nil.

Just because the snapshot target doesn't exist doesn't mean that the
snapshot cache should be nil. This makes serialization easier (one less
nil check if snapshot error is non-nil), and just makes more sense
conceptually (if there is no error, all other return values are valid).

havoc-io/mutagen,havoc-io/mutagen