`Gitlab::Ci::Trace::Stream#path` should return `nil` when the trace is in ObjectStorage
Summary
@ayufan is expecting the following implementation,
# If trace is in FileStorage, then return local path.
# If trace is in ObjectStorage, then return nil.
Gitlab::Ci::Trace::Stream#path
# If trace is in FileStorage, then return nil.
# If trace is in ObjectStorage, then return remote path.
Gitlab::Ci::Trace::Stream#url
https://gitlab.slack.com/archives/C8QK62E76/p1518446954000628
Current implementation
# If trace is in FileStorage, then return local path.
# If trace is in ObjectStorage, then return remote path.
Gitlab::Ci::Trace::Stream#path
# NoMethodError: undefined method `url
Gitlab::Ci::Trace::Stream#url
/cc @ayufan