Minor cleanup

app/src/main/java/eu/kanade/tachiyomi/ui/browse/extension/details/ExtensionDetailsScreenModel.kt

@@ -173,7 +173,7 @@ data class ExtensionDetailsState(
 ) {
 
     val sources: List<ExtensionSourceItem>
-        get() = _sources ?: emptyList()
+        get() = _sources.orEmpty()
 
     val isLoading: Boolean
         get() = extension == null || _sources == null

app/src/main/java/eu/kanade/tachiyomi/ui/browse/migration/manga/MigrationMangaScreenModel.kt

@@ -64,7 +64,7 @@ data class MigrateMangaState(
 ) {
 
     val titles: List<Manga>
-        get() = titleList ?: emptyList()
+        get() = titleList.orEmpty()
 
     val isLoading: Boolean
         get() = source == null || titleList == null

app/src/main/java/eu/kanade/tachiyomi/ui/browse/source/browse/BrowseSourceScreenModel.kt

@@ -311,7 +311,7 @@ class BrowseSourceScreenModel(
         return getCategories.subscribe()
             .firstOrNull()
             ?.filterNot { it.isSystemCategory }
-            ?: emptyList()
+            .orEmpty()
     }
 
     suspend fun getDuplicateLibraryManga(manga: Manga): Manga? {