cmd/fsck-cmd.py: Do not warn about empty par2 output

cmd/fsck-cmd.py: Do not warn about empty par2 output

With a recent `par2` installed, `bup fsck -v` outputs messages like

  Unexpected par2 error output
  ''Assuming par2 supports parallel processing

when there was no error output and the exit status was zero.  Stop
printing the warning in that case.

Signed-off-by: Christian Cornelssen <ccorn@1tein.de>
[rlb@defaultvalue.org: add information from the pr to commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>