caps: always send CAP END after CAP LS, even if no caps are requested
Fixes https://bugs.kde.org/show_bug.cgi?id=471940
Konversation doesn't send CAP END after CAP LS, unless it actually requests caps with CAP REQ. As per https://ircv3.net/specs/extensions/capability-negotiation.html it must send CAP END - otherwise servers will just hang the connection and never complete the initial registration.