`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