Avoid creating a row if the `TableEditor.row_factory` callable returns None

Feature request.

Use case: when the user right clicks on a `TableEditor` and selects "Add new item", I would like to open a dialog where the user can customize the new object before it gets added to the table. However, I would also like the user the option to dismiss the creation.

Currently, if I make the `row_factory` return `None`, a row of _None_ will be added. If I make it `raise` on dismissal, a traceback will be leaked to the console.

```python
from traits.api import HasStrictTraits, Str, Int, List, Instance
from traitsui.api import TableEditor, UItem, View
from pyface.api import choose_one

class Vehicle(HasStrictTraits):
name = Str()
type = Str()
year = Int(2022)

def initialize_vehicle():
""" Returns a Vehicle unless the user dismisses, in which case returns None"""
type_ = choose_one(None, "type", ["bike", "car", "scooter"])
if type_ is None:
return None
return Vehicle(type=type_)

class Garage(HasStrictTraits):
vehicles = List(Instance(Vehicle))
traits_view = View(
UItem(
"vehicles",
editor=TableEditor(
row_factory=initialize_vehicle,
sortable=False
)
), width=300, height=180
)

Garage(vehicles=[Vehicle(name="Lalaa", type="bike", year=1999)]).configure_traits()
```