ListStrEditor does not respect `auto_add=True` when using Qt

When using the `auto_add=True` option an empty item is expected to be visible at the end of the list for the user to edit and append to the list trait that the ListStrEditor edits. However, setting `auto_add=True` does not look that it has any effect.

To replicate use the example
https://github.com/enthought/traitsui/blob/master/examples/demo/Advanced/ListStrEditor_demo.py

The problematic behaviour is present using qt (pyside) while it is working fine using wxpython