fix: LineChartDataPoint.tooltip not properly rendered

[ndonkoHenri]

Fixes #4967

Test Code

import flet as ft

def main(page: ft.Page):
    chart = ft.LineChart([ft.LineChartData(
        data_points=[
            ft.LineChartDataPoint(0, 0),
            ft.LineChartDataPoint(1, 0, tooltip="BUG"),
            ft.LineChartDataPoint(2, 0, tooltip=None),
        ],
        point=True,
    )])
    page.add(chart)
    page.add(ft.Text('HELLO WORLD'))

ft.app(main)

Summary by Sourcery

Bug Fixes:

• Fixes an issue where the tooltip for LineChartDataPoint was not rendering correctly.