print everything except progress in non-terminal

Instead of only checking `Progress != nil` (which is always true because
the server sends `"progressDetail":{}` when it doesnt have the progress), we
also check if `Progress.String() != ""`, which should be sufficient to filter
out the progress data.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Merge pull request #8150 from dqminh/pull-output-non-terminal

Pull: print some data instead of newlines when output is not a terminal