fix #3919: log view: show first task output line correctly

fix #3919: log view: show first task output line correctly

if a task did not produce output yet, we always get a single line
with "no output". our heuristic in the gui counts the total lines +
the current  position. so to update the first output correctly, we
have to update every time in case we only have one line.

Otherwise, we only update on the second line, which is bad
in case the only line we ever get is the result.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>