Allow SyncResponse state events to be optional

[ouchadam]

• dendrite can omit the state field from the sync response

19:53:34.590  W  Caused by: kotlinx.serialization.MissingFieldException: Field 'state' is required for type with serial name 'app.dapk.st.matrix.sync.internal.request.ApiSyncRoom', but it was missing
19:53:34.591  W  	at kotlinx.serialization.internal.PluginExceptionsKt.throwMissingFieldException(PluginExceptions.kt:20)
19:53:34.591  W  	at app.dapk.st.matrix.sync.internal.request.ApiSyncRoom.<init>(ApiSyncResponse.kt:53)
19:53:34.591  W  	at app.dapk.st.matrix.sync.internal.request.ApiSyncRoom$$serializer.deserialize(ApiSyncResponse.kt:53)
19:53:34.591  W  	at app.dapk.st.matrix.sync.internal.request.ApiSyncRoom$$serializer.deserialize(ApiSyncResponse.kt:53)
19:53:34.591  W  	at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:70)
19:53:34.591  W  	... 65 more

[github-actions]

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