[KPM] Suppress unused serialVersionUID in Empty

KT-51386

libraries/tools/kotlin-gradle-plugin-idea/src/main/kotlin/org/jetbrains/kotlin/gradle/kpm/KotlinExternalModelContainer.kt

@@ -39,7 +39,8 @@ sealed class KotlinExternalModelContainer : Serializable {
         override fun <T : Any> contains(key: KotlinExternalModelKey<T>): Boolean = false
         override fun <T : Any> get(key: KotlinExternalModelKey<T>): T? = null
 
-
+        /* Necessary for stable serialization */
+        @Suppress("unused")
         private const val serialVersionUID = 0L
     }