core: Fix CAP REQ display for SASL only supported

core: Fix CAP REQ display for SASL only supported

Modify queuedCapsDisplay to only add a ", " when both
_capsQueuedIndividual and _capsQueuedBundled are not empty.  This
avoids adding a spurious ", " when there's only individual caps, e.g.
SASL.  Though the docs don't mention it, QStringList.join() will only
add the separator when there's more than one item in the list.

Before:
* Negotiating capabilities (requesting: sasl, )...
After:
* Negotiating capabilities (requesting: sasl)...

See https://doc.qt.io/qt-5/qstringlist.html#join

Closes GH-334.