qdoc: qhp group: selector does not work for groups without a \group page

QDoc supports grouping items/pages with the \ingroup command, and lists of group members can be then generated e.g. with the \generatelist <group_name> command. This works even if the group is not documented with the \group command.

Likewise, the qhp subproject definition in .qdocconf supports the group:group_name selector. But here, the selector works only if the group is documented, that is, a \group page exists for it.