This matches the GNOME implementation as well as the openconnect binary's
default behavior when "--protocol" is not specified.

Connections created via nmcli, for example, can be set up without explicitly
passing "protocol" in the VPN data; this is expected to be interpreted as
the AnyConnect protocol rather than Pulse.

While here, also check NM_OPENCONNECT_KEY_REPORTED_OS's presence via
QMap::contains() rather than creating and comparing to an empty QString.