fix: display good avatar in case of missing author

src/components/Feed/Banner.vue

@@ -3,7 +3,7 @@
 		<img class="background" :src="imageUrl">
 		<div class="content">
 			<NcAvatar class="avatar"
-				:display-name="author"
+				:display-name="author || title"
 				:is-no-user="true"
 				:size="128"
 				:url="imageUrl" />

src/components/Sidebar/Item.vue

@@ -4,7 +4,7 @@
 		:to="toUrl(url)">
 		<template #icon>
 			<NcAvatar v-if="feed"
-				:display-name="feed.author"
+				:display-name="feed.author || feed.title"
 				:is-no-user="true"
 				:url="feed.imageUrl" />
 			<Alert v-if="failed" />