apiDiscordRoutes fix.

- explicitNulls = false

webserver/src/main/kotlin/space/mori/chzzk_bot/webserver/ktorClient.kt

@@ -2,13 +2,16 @@ import io.ktor.client.*
 import io.ktor.client.engine.cio.*
 import io.ktor.client.plugins.contentnegotiation.*
 import io.ktor.serialization.kotlinx.json.*
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.json.Json
 
+@OptIn(ExperimentalSerializationApi::class)
 val applicationHttpClient = HttpClient(CIO) {
     install(ContentNegotiation) {
         json(json = Json {
             ignoreUnknownKeys = true
             coerceInputValues = true
+            explicitNulls = false
         })
     }
 }