DBus::Connection::signal_subscribe(): Pass nullptr instead of "".
DBus::Connection::signal_subscribe(): Pass nullptr instead of "".

Because that's what the C documentation requires, and the implementation
doesn't accept "" to mean the same thing, though that would be
convenient:
https://developer.gnome.org/gio/stable/GDBusConnection.html#g-dbus-connection-signal-subscribe

Noticed by Christof Meerwald on gtkmm-list.

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