fix: make field truly optional

[DominicGBauer]

Description

I saw this issue appear now that we fixed allowing write_checkpoint to be nullable.

SyncStream::streamingSync Error: kotlinx.serialization.MissingFieldException: Field 'write_checkpoint' is required for type with serial name 'com.powersync.sync.StreamingSyncCheckpointDiff', but it was missing

Work Done

• Made write_checkpoint = null so that it is truly optional as explained in https://stackoverflow.com/questions/64796913/kotlinx-serialization-missingfieldexception.

Note we could also implement this system wide to avoid similar issues in future https://github.com/Kotlin/kotlinx.serialization/blob/v1.3.0-RC/docs/json.md#explicit-nulls

[DominicGBauer]

@Manrich121 Steven asked if you could comment on this please :) w.r.t. to setting explicitNull

[Manrich121]

@Manrich121 Steven asked if you could comment on this please :) w.r.t. to setting explicitNull

This might also be relevant discussion #5 (comment)

So providing a default value there should be fine. Configuring JsonUtil to always expect explicit nulls, might impact things unexpectedly