fix: Sometimes url isn't even defined.

shared/src/commonMain/kotlin/bou/amine/apps/readerforselfossv2/model/MercuryModel.kt

@@ -9,6 +9,9 @@ class MercuryModel {
         val title: String?,
         val content: String?,
         val lead_image_url: String?, // NOSONAR
-        val url: String
+        val url: String?,
+        val error: Boolean?,
+        val message: String?,
+        val failed: Boolean?
     )
 }