Change serialization of SyncFrom to NULL

[taco-indenbosch]

I changed the serialization of the SyncFrom property from Javascript's undefined to null as required by the API. Without it new synchronized blocks cannot be created.

Description

The NullValueHandling = NullValueHandling.Include argument to the JsonProperty attribute ensures that .NET null values are serialized as Javascript null values instead of undefined. The Notion API requires the sync_from property to be null for new synced blocks.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I am using the Notion.NET library to upload converted data to Notion. Since this change I have been able to create synced blocks, both new and linking to existing ones.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

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