Fix colored markup drawing in Mac wxDataViewCtrl

50ba73c accidentally omitted an if (colText) check, causing calls to
setTextColor:nil - which in turn caused markup text (but curiously, not
plain) rendering to stick with a previously set color even on cells
where the default color should be used.

Restoring the original logic to prevent setTextColor:nil fixes it.

src/osx/cocoa/dataview.mm

@@ -2940,7 +2940,9 @@ void wxDataViewRenderer::SetAttr(const wxDataViewItemAttr& attr)
     {
         if ( !colText )
             colText = data->GetOriginalTextColour();
-        [(id)cell setTextColor:colText];
+
+        if ( colText )
+            [(id)cell setTextColor:colText];
     }
 
     if ( [cell respondsToSelector:@selector(setDrawsBackground:)] )