DateTimePicker support for optional on selectedDate

[agiguere]

SDK 25.8.0

The DateTimePicker view should support nil value for date selection, we must allow the user to select a date an not default an initial one

property selectedDate, convert to an optional binding

same design as the UIkit equivalent API

https://help.sap.com/doc/978e4f6c968c4cc5a30f9d324aa4b1d7/latest/en-US/Documents/Frameworks/SAPFiori/Classes/FUIDatePickerFormCell.html#/s:8SAPFiori21FUIDatePickerFormCellC5value10Foundation4DateVSgvp

thanks

[hengyi-zhang]

The enhancement had already been merged into main branch: #1205