Added changes in date picker for DateRangePicker

[IchinoseBinit]

The previous code snippet did not allow selecting an initial date past the end date. Also, there was an issue with the null check where the method was called after setting the value to null with the argument passed with a null check.

I have resolved those issues for DateTimeRangePicker.

[sarbagyastha]

Hi @IchinoseBinit, thanks for the PR. Can you mention your use case, why do you want initial date past end date?

[sarbagyastha]

The second issue was a blunder.

Could you please make the onEndDateChanged, ValueChanged<NepaliDateTime?> and remove all the bang operators from end dates that's being passed to the callback instead.

[IchinoseBinit]

The first case has a use case like when a user tries to change the selected date after a certain start and end date has been selected by mistake or the users wishes to change the start and end date. It is useful as the first selection of the user might not always be a correct one.