Parse nullable properties with non-default values when the keys are not present in the Json

What is your use-case and why do you need this feature?

Ability to parse nullable properties when the keys are not present in the JSON.

@kotlinx.serialization.Serializable
data class Foo(val foo: String?)

Trying to parse this JSON string {} fails.

The current workaround would be to add a default value val foo: String? = null. However, we don’t always have access to the entities in order to apply the fix. One example is when trying to create external serializers for the SqlDelight autogenerated entities.

Describe the solution you’d like Add a configuration acceptNonPresentKeys which when set to true, the parser would automatically produce Foo(foo = null) when {} this is provided.

[kingsleyadio]

Resurfacing this issue again I also just got bit by the same problem while trying to migrate from Moshi to kotlinx.serialization I had nullable properties that I expected to be initialized with null if the value is ever missing from the API, but instead, the app just crashed I believe it would be really nice to provide this option as an opt-in configuration, so whoever uses it is fully aware and there won’t be any confusion. This will also make transition to kotlinx.serialization from other libraries much smoother

[shanshin]

see #1535 Added Json { explicitNulls = false } flag, target version is 1.3.0