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

[Murray Cumming <murrayc src gnome org>]

commit ece1463f6d796c7c74da1e9d37a4e69904a9706e
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Sep 21 08:44:05 2015 +0200

    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.

 gio/src/dbusconnection.ccg |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/gio/src/dbusconnection.ccg b/gio/src/dbusconnection.ccg
index 0e9faf6..935a5d6 100644
--- a/gio/src/dbusconnection.ccg
+++ b/gio/src/dbusconnection.ccg
@@ -807,8 +807,12 @@ guint Connection::signal_subscribe(
 {
   auto slot_copy = new SlotSignal(slot);
 
-  return g_dbus_connection_signal_subscribe(gobj(), sender.c_str(),
-    interface_name.c_str(), member.c_str(), object_path.c_str(), arg0.c_str(),
+  return g_dbus_connection_signal_subscribe(gobj(),
+    (sender.empty() ? nullptr : sender.c_str()),
+    (interface_name.empty() ? nullptr : interface_name.c_str()),
+    (member.empty() ? nullptr : member.c_str()),
+    (object_path.empty() ? nullptr : object_path.c_str()),
+    (arg0.empty() ? nullptr : arg0.c_str()),
     static_cast<GDBusSignalFlags>(flags),
     &DBusConnection_Signal_giomm_callback, slot_copy,
     &DBusConnection_Signal_giomm_callback_destroy);