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 31f7bf1994 ("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

src/shared/logs-show.c

@@ -124,6 +124,11 @@ static bool print_multiline(FILE *f, unsigned prefix, unsigned n_columns, Output
                 }
         }
 
+        /* A special case: make sure that we print a newline when
+           the message is empty. */
+        if (message_len == 0)
+                fputs("\n", f);
+
         for (pos = message;
              pos < message + message_len;
              pos = end + 1, line++) {