refactor value setting for javascript variables (#7906)

When retrieving an unset shortcut value, the error is not silenced but
instead we use the null-coalesce operator to define a default value.

See #7290

app/views/helpers/javascript_vars.phtml

@@ -36,30 +36,30 @@ echo json_encode([
 		'version' => FRESHRSS_VERSION,
 	],
 	'shortcuts' => [
-		'actualize' => @$s['actualize'],
-		'mark_read' => @$s['mark_read'],
-		'mark_favorite' => @$s['mark_favorite'],
-		'go_website' => @$s['go_website'],
-		'prev_entry' => @$s['prev_entry'],
-		'next_entry' => @$s['next_entry'],
-		'next_unread_entry' => @$s['next_unread_entry'],
-		'skip_prev_entry' => @$s['skip_prev_entry'],
-		'skip_next_entry' => @$s['skip_next_entry'],
-		'first_entry' => @$s['first_entry'],
-		'last_entry' => @$s['last_entry'],
-		'collapse_entry' => @$s['collapse_entry'],
-		'load_more' => @$s['load_more'],
-		'mylabels' => @$s['mylabels'],
-		'auto_share' => @$s['auto_share'],
-		'focus_search' => @$s['focus_search'],
-		'user_filter' => @$s['user_filter'],
-		'help' => @$s['help'],
-		'close_menus' => @$s['close_menus'],
-		'normal_view' => @$s['normal_view'],
-		'global_view' => @$s['global_view'],
-		'reading_view' => @$s['reading_view'],
-		'rss_view' => @$s['rss_view'],
-		'toggle_media' => @$s['toggle_media'],
+		'actualize' => $s['actualize'] ?? '',
+		'mark_read' => $s['mark_read'] ?? '',
+		'mark_favorite' => $s['mark_favorite'] ?? '',
+		'go_website' => $s['go_website'] ?? '',
+		'prev_entry' => $s['prev_entry'] ?? '',
+		'next_entry' => $s['next_entry'] ?? '',
+		'next_unread_entry' => $s['next_unread_entry'] ?? '',
+		'skip_prev_entry' => $s['skip_prev_entry'] ?? '',
+		'skip_next_entry' => $s['skip_next_entry'] ?? '',
+		'first_entry' => $s['first_entry'] ?? '',
+		'last_entry' => $s['last_entry'] ?? '',
+		'collapse_entry' => $s['collapse_entry'] ?? '',
+		'load_more' => $s['load_more'] ?? '',
+		'mylabels' => $s['mylabels'] ?? '',
+		'auto_share' => $s['auto_share'] ?? '',
+		'focus_search' => $s['focus_search'] ?? '',
+		'user_filter' => $s['user_filter'] ?? '',
+		'help' => $s['help'] ?? '',
+		'close_menus' => $s['close_menus'] ?? '',
+		'normal_view' => $s['normal_view'] ?? '',
+		'global_view' => $s['global_view'] ?? '',
+		'reading_view' => $s['reading_view'] ?? '',
+		'rss_view' => $s['rss_view'] ?? '',
+		'toggle_media' => $s['toggle_media'] ?? '',
 	],
 	'urls' => [
 		'index' => _url('index', 'index'),