Make ControlEvent.data of type Optional[str] with None as default

[ndonkoHenri]

Resolves #4786

Test Code

import flet as ft


def main(page: ft.Page):
    page.add(
        ft.TextField(
            label="Hello",
            on_click=lambda e: print(f"Clicked: {e.data!r}"),  # None
            on_focus=lambda e: print(f"Focused: {e.data!r}"),  # None
            on_change=lambda e: print(f"Changed: {e.data!r}"),  # String
        ),
    )


ft.app(target=main)

Additional details

Complementary PRs:

• eventData parameter in backend.triggerControlEvent accepts null values flet-video#2

Summary by Sourcery

Make ControlEvent.data optional.

Bug Fixes:

• Fix issue where ControlEvent.data was always a string, even when no data was provided.

Enhancements:

• Improve type handling in ControlEvent by making data field optional.