[WebUI] Fix case when no cluster is defined
[WebUI] Fix case when no cluster is defined

Issue: #1334
Reported by: @dehnli

AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd