tdf#116334: Actually when there is no handler, we have to return 'true'.

Also use the return value of the link's Call() [as the SvTreeListBox::ExpandingHdl()
is doing], the appropriate callbacks seem to return the expected 'false' in
the cases I've reviewed...

Change-Id: I0cdd63e8ec4c794839070b914150e0b32f743359
Reviewed-on: https://gerrit.libreoffice.org/51211
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>

svtools/source/contnr/treelistbox.cxx

@@ -427,8 +427,7 @@ void SvTreeListBox::DeselectHdl()
 
 bool SvTreeListBox::DoubleClickHdl()
 {
-    aDoubleClickHdl.Call( this );
-    return false;
+    return !aDoubleClickHdl.IsSet() || aDoubleClickHdl.Call(this);
 }