add @nullable to default fields, update libpray_example.dart to refle…

[mumer92]

Added @nullable to default fields, update libpray_example.dart to reflect Changes with default values provided for calculation

[mirmahfuz99]

Change prayer_calculation_settings like this :+1: https://gist.github.com/mirmahfuz99/ecd79497bc425221970948bbd31e2949 And Change prayer example like this: https://gist.github.com/mirmahfuz99/3fbdaa899cf24e9ac86b507fb88b2415

From mumer92 . Thanks mumer92.