Don't send outbuf to Rack input #read if it's nil
Don't send outbuf to Rack input #read if it's nil

JRuby's IO.copy_stream implementation doesn't use a buffer string when
calling #read, which means that we're passing nil as the second
argument, and Rack::Link complains about that. Instead we should simply
ignore both length and outbuf arguments if they're nil.

janko-m/tus-ruby-server