fix(gateway): properly deserialise null, such as in HeartbeatAck

core/src/commonMain/kotlin/moe/lava/neon/core/api/gateway/SerializingExtensions.kt

@@ -1,6 +1,7 @@
 package moe.lava.neon.core.api.gateway
 
 import kotlinx.serialization.KSerializer
+import kotlinx.serialization.json.JsonNull
 import kotlinx.serialization.json.decodeFromJsonElement
 import moe.lava.neon.core.api.ApiConstants
 
@@ -32,7 +33,7 @@ fun Payload.Unknown.asIncoming() : Payload.WithSequence {
 private inline fun <reified T : Event.Incoming> Payload.Unknown.decode(): Payload.Incoming<T> =
     Payload.Incoming(
         op = op,
-        d = json.decodeFromJsonElement<T>(d!!),
+        d = json.decodeFromJsonElement<T>(d ?: JsonNull),
         s = s,
         t = t,
     )