Avoid crash when trying to open random entry but categories are still loading

Fixes #9610

app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryScreenModel.kt

@@ -527,6 +527,8 @@ class LibraryScreenModel(
     }
 
     suspend fun getRandomLibraryItemForCurrentCategory(): LibraryItem? {
+        if (state.value.categories.isEmpty()) return null
+
         return withIOContext {
             state.value
                 .getLibraryItemsByCategoryId(state.value.categories[activeCategoryIndex].id)