book library Null safety

Classes

Book
An object which handle the details of a book
DateConverter
A json converter that handles the date_published field of the API It can be either a year, a full date... so we need to handle this properly to return always the same type.
MsrpConverter
A json converter that handles the msrp field of the API It can be either a string, an int... so we need to handle this properly to return always the same type.