fix: Make KotlinSerializer.kt more robust

[geirsagberg]

Add handling of null, Void and Object

Brief, plain english overview of your changes here

We received errors when null values, Void values and plain Objects were attempted serialized/deserialized. These changes fix the errors for us.

Fixes

Reminders

• [x] Added/ran automated tests
• [x] Update README and/or examples
• [x] Ran mvn spotless:apply

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cc @kagkarlsson