fix: crash when importing subscriptions list with invalid avatars (#5931

)

Solves issue of subscription list not loading when channels have old/invalid avatars.

app/src/main/java/com/github/libretube/api/obj/Subscription.kt

@@ -6,6 +6,6 @@ import kotlinx.serialization.Serializable
 data class Subscription(
     val url: String,
     val name: String,
-    val avatar: String,
+    val avatar: String? = null,
     val verified: Boolean
 )