Skip drawing empty AUI panes.

This seems to be unnecessary and generates GTK+ warnings as it results in
passing rectangle with negative (after accounting for padding/margins) size to
gtk_paint_box() in wxAuiGtkTabArt, as could be seen e.g. when closing
"wxTextCtrl 2" tab in the aui sample.

(cherry picked from commit 1f16829c01)

src/aui/framemanager.cpp

@@ -3854,7 +3854,12 @@ void wxAuiManager::OnRender(wxAuiManagerEvent& evt)
         wxAuiDockUIPart& part = m_uiParts.Item(i);
 
         // don't draw hidden pane items or items that aren't windows
-        if (part.sizer_item && ((!part.sizer_item->IsWindow() && !part.sizer_item->IsSpacer() && !part.sizer_item->IsSizer()) || !part.sizer_item->IsShown()))
+        if (part.sizer_item &&
+                ((!part.sizer_item->IsWindow() &&
+                  !part.sizer_item->IsSpacer() &&
+                  !part.sizer_item->IsSizer()) ||
+                  !part.sizer_item->IsShown() ||
+                   part.rect.IsEmpty()))
             continue;
 
         switch (part.type)