logs-show: fix corrupt output with empty messages

logs-show: fix corrupt output with empty messages

If a message had zero length, journalctl would print no newline, and
two output lines would be concatenated. Fix. The problem was
introduced in commit 31f7bf199452 ("logs-show: print multiline
messages"). Affected short and verbose output modes.

Before fix:

Feb 09 21:16:17 glyph dhclient[1323]: Feb 09 21:16:17 glyph NetworkManager[788]: <info> (enp4s2): DHCPv4 state changed nbi -> preinit

after:

Feb 09 21:16:17 glyph dhclient[1323]:
Feb 09 21:16:17 glyph NetworkManager[788]: <info> (enp4s2): DHCPv4 state changed nbi -> preinit