Always create an empty vardict instead of passing NULL

In the error case, we will clear the GVariant and then try to emit a
response from a NULL pointer as result. Always create an empty vardict
when something failed instead.

This also clears the error because we continue on in the error case and
do in fact re-using it in one case.

src/input-capture.c

@@ -473,16 +473,17 @@ set_pointer_barriers_done (GObject      *source_object,
                                                                      &results,
                                                                      res,
                                                                      &error))
-  {
+    {
       g_dbus_error_strip_remote_error (error);
       g_warning ("A backend call failed: %s", error->message);
+      g_clear_error (&error);
     }
 
   should_close_session = !request->exported || response != 0;
 
   if (request->exported)
     {
-      if (response != 0)
+      if (!results)
         {
           GVariantBuilder results_builder;
 

src/remote-desktop.c

@@ -383,6 +383,7 @@ select_devices_done (GObject *source_object,
     {
       g_dbus_error_strip_remote_error (error);
       g_warning ("A backend call failed: %s", error->message);
+      g_clear_error (&error);
     }
 
   should_close_session = !request->exported || response != 0;
@@ -664,6 +665,7 @@ start_done (GObject *source_object,
     {
       g_dbus_error_strip_remote_error (error);
       g_warning ("A backend call failed: %s", error->message);
+      g_clear_error (&error);
     }
 
   should_close_session = !request->exported || response != 0;
@@ -682,7 +684,8 @@ start_done (GObject *source_object,
               should_close_session = TRUE;
             }
         }
-      else
+
+      if (!results)
         {
           GVariantBuilder results_builder;