Fix showing cells without values in wxDataViewCtrl

Fix regression introduced in a49567109a: cells
without value, i.e. for which the wxVariant returned from GetValue() is not
set at all, should appear as empty, not reuse the last value used by this
renderer.

src/common/datavcmn.cpp

@@ -813,12 +813,14 @@ wxDataViewRendererBase::PrepareForItem(const wxDataViewModel *model,
                                        const wxDataViewItem& item,
                                        unsigned column)
 {
-    // Now check if we have a value and remember it if we do.
+    // Now check if we have a value and remember it for rendering it later.
+    // Notice that we do it even if it's null, as the cell should be empty then
+    // and not show the last used value.
     const wxVariant& value = CheckedGetValue(model, item, column);
+    SetValue(value);
+
     if ( !value.IsNull() )
     {
-        SetValue(value);
-
         // Also set up the attributes for this item if it's not empty.
         wxDataViewItemAttr attr;
         model->GetAttr(item, column, attr);