DBus::Connection: Pass an empty bus name as nullptr

* gio/src/dbusconnection.[hg|ccg]: Some string parameters in some C functions
accept nullptr, others don't.
call(), call_sync(): bus_name: Pass nullptr for an empty string.
emit_signal(): Only destination_bus_name can be nullptr.
               Other string parameters: Pass c_str().
The bug in call() and call_sync() was noticed by Gerlof Fokkema on gtkmm-list.
The bug in emit_signal() is an overreaction to bug 645072. All string
parameters were changed to pass nullptr, but only destination_bus_name
should have been changed.