Show default message when no categories selected

app/src/main/java/eu/kanade/tachiyomi/ui/setting/SettingsGeneralFragment.kt

@@ -84,10 +84,16 @@ class SettingsGeneralFragment : SettingsFragment(),
 
         subscriptions += preferences.libraryUpdateCategories().asObservable()
                 .subscribe {
-                    categoryUpdate.summary = it
+                    val selectedCategories = it
                             .mapNotNull { id -> dbCategories.find { it.id == id.toInt() } }
                             .sortedBy { it.order }
-                            .joinToString { it.name }
+
+                    val summary = if (selectedCategories.isEmpty())
+                        getString(R.string.all)
+                    else
+                        selectedCategories.joinToString { it.name }
+
+                    categoryUpdate.summary = summary
                 }
 
         themePreference.setOnPreferenceChangeListener { preference, newValue ->

app/src/main/res/values/strings.xml

@@ -86,6 +86,7 @@
     <string name="update_24hour">Daily</string>
     <string name="update_48hour">Every 2 days</string>
     <string name="pref_library_update_categories">Categories to include in global update</string>
+    <string name="all">All</string>
     <string name="pref_library_update_restriction">Library update restrictions</string>
     <string name="pref_library_update_restriction_summary">Update only when the conditions are met</string>
     <string name="wifi">Wi-Fi</string>