shared/logs-show: avoid printing "(null)" when timestamp is not specified

shared/logs-show: avoid printing "(null)" when timestamp is not specified

$ perl -e 'print("MESSAGE\n", pack("q<", 1), "A\n\nMESSAGE=test2\n")' > message.bin
$ systemd-journal-remote -o /tmp/out.journal message.bin
$ journalctl -o export --file /tmp/out.journal
__CURSOR=s=b16c464c2db44384b29e75a564d8388e;i=1;b=6b0be47627bd4932913dc126012c21c0;m=0;t=0;x=b04263a253e357a
__REALTIME_TIMESTAMP=0
__MONOTONIC_TIMESTAMP=0
_BOOT_ID=6b0be47627bd4932913dc126012c21c0
MESSAGE=A

$ journalctl -o verbose --file /tmp/out.journal
(null) [s=b16c464c2db44384b29e75a564d8388e;i=1;b=6b0be47627bd4932913dc126012c21c0;m=0;t=0;x=b04263a253e357a]
    MESSAGE=A

This is changed to
$ build/journalctl -o verbose --file /tmp/out.journal
(no timestamp) [s=b16c464c2db44384b29e75a564d8388e;i=1;b=6b0be47627bd4932913dc126012c21c0;m=0;t=0;x=b04263a253e357a]
    MESSAGE=A

We should deal gracefully with unexpected input.