logrotate: Do not rotate empty logs

As a useful side effect, prevents "Unable to contact server. Is it
running?" mails from cron when fail2ban hasn't been (intentionally)
running nor thus logging anything either.

ChangeLog

@@ -18,6 +18,7 @@ ver. 0.9.4 (2015/XX/XXX) - wanna-be-released
 - New Features:
 
 - Enhancements:
+   * Do not rotate empty log files
 
 ver. 0.9.3 (2015/08/01) - lets-all-stay-friends
 ----------

files/fail2ban-logrotate

@@ -8,6 +8,7 @@
 /var/log/fail2ban.log {
     rotate 7
     missingok
+    notifempty
     compress
     postrotate
       /usr/bin/fail2ban-client flushlogs  1>/dev/null || true