fix(models): make created and ownedBy nullable (#332)

Author: thiswillbeyourgithub

openai-core/src/commonMain/kotlin/com.aallam.openai.api/model/Model.kt

@@ -9,7 +9,7 @@ import kotlinx.serialization.Serializable
 @Serializable
 public data class Model(
     @SerialName("id") public val id: ModelId,
-    @SerialName("created") public val created: Long,
-    @SerialName("owned_by") public val ownedBy: String,
+    @SerialName("created") public val created: Long? = null,
+    @SerialName("owned_by") public val ownedBy: String? = null,
     @SerialName("permission") public val permission: List<ModelPermission>? = null,
 )