customDayBuilder is broken

[trygvis]

Version of flutter_calendar_carousel

2.0.0

Expected behavior

After #260 customDayBuilder can not return null. There is also no way to return the default implementation which could have been an alternative, but as it's nullability is documented as a feature I suspect this happened by accident. Link to diff

I haven't looked at the other custom builders, but being able to return null if one doesn't want to customize the widget is useful. We want to customize the current day, and have the default behavior for all other days.

Suggested new implementation of getDayContainer():

Widget getDayContainer(
    bool isSelectable,
    int index,
    bool isSelectedDay,
    bool isToday,
    bool isPrevMonthDay,
    TextStyle? textStyle,
    TextStyle defaultTextStyle,
    bool isNextMonthDay,
    bool isThisMonthDay,
    DateTime now) {
  final customDayBuilder = widget.customDayBuilder;
  Widget? widget;
  if (customDayBuilder != null) {
    final TextStyle appTextStyle = DefaultTextStyle.of(context).style;
    TextStyle styleForBuilder = appTextStyle.merge(getDayStyle(
        isSelectable,
        index,
        isSelectedDay,
        isToday,
        isPrevMonthDay,
        textStyle,
        defaultTextStyle,
        isNextMonthDay,
        isThisMonthDay));

    widget = customDayBuilder(isSelectable, index, isSelectedDay, isToday,
        isPrevMonthDay, styleForBuilder, isNextMonthDay, isThisMonthDay, now);
  }

  return widget ?? getDefaultDayContainer(
      isSelectable,
      index,
      isSelectedDay,
      isToday,
      isPrevMonthDay,
      textStyle,
      defaultTextStyle,
      isNextMonthDay,
      isThisMonthDay,
      now);
}