Added @nullable to some property & modified example

[bhangun]

• Added @nullable i got this when some PrayerCalculationSettings set or has null value. So i added @nullable to some property for null safety.

  Tried to build class "PrayerCalculationSettings" but nested builder for field "imsakParameter" 
  threw: Tried to construct class "PrayerCalculationParameter" with null field "value". 
  This is forbidden; to allow it, mark "value" with @nullable.

• Modified Example I was suffer with existing example which PrayerCalculationSettingsBuilder doesn't override default value so it cause null value. So i copy default value to the example. Hope others could get better experience when using this example library.