18104c2 coreos: fix returned streamData during refreshStream

Authored and Committed by rfairley 4 years ago
    coreos: fix returned streamData during refreshStream
    
    If streamData is an empty object, set it to `null` so that
    `Loading...` text will be displayed in the case of an
    unsuccessful fetch, rather than render empty fields for
    artifacts.
    
    Co-authored-by: Allen Bai <abai@redhat.com>
    Signed-off-by: Robert Fairley <rfairley@redhat.com>