Fix NPE when avatarUrl is empty

app/src/main/java/org/schabi/newpipe/local/feed/service/FeedUpdateInfo.kt

@@ -18,7 +18,7 @@ data class FeedUpdateInfo(
     @NotificationMode
     val notificationMode: Int,
     val name: String,
-    val avatarUrl: String,
+    val avatarUrl: String?,
     val url: String,
     val serviceId: Int,
     // description and subscriberCount are null if the constructor info is from the fast feed method

app/src/main/java/org/schabi/newpipe/local/subscription/SubscriptionManager.kt

@@ -100,7 +100,9 @@ class SubscriptionManager(context: Context) {
         val subscriptionEntity = subscriptionTable.getSubscription(info.uid)
 
         subscriptionEntity.name = info.name
-        subscriptionEntity.avatarUrl = info.avatarUrl
+
+        // some services do not provide an avatar URL
+        info.avatarUrl?.let { subscriptionEntity.avatarUrl = it }
 
         // these two fields are null if the feed info was fetched using the fast feed method
         info.description?.let { subscriptionEntity.description = it }