Fix OPML export (#5238)

Regression from https://github.com/FreshRSS/FreshRSS/pull/4403

app/views/helpers/export/opml.phtml

@@ -79,7 +79,7 @@ $opml_array = [
 ];
 
 if (!empty($this->categories)) {
-	foreach ($this->categories as $key => $cat) {
+	foreach ($this->categories as $cat) {
 		$outline = [
 			'text' => htmlspecialchars_decode($cat->name(), ENT_QUOTES),
 			'@outlines' => feedsToOutlines($cat->feeds(), $this->excludeMutedFeeds),
@@ -89,12 +89,12 @@ if (!empty($this->categories)) {
 			$outline['frss:opmlUrl'] = $cat->attributes('opml_url');
 		}
 
-		$opml_array['body'][$key] = $outline;
+		$opml_array['body'][] = $outline;
 	}
 }
 
 if (!empty($this->feeds)) {
-	$opml_array['body'][] = feedsToOutlines($this->feeds, $this->excludeMutedFeeds);
+	$opml_array['body'] = array_merge($opml_array['body'], feedsToOutlines($this->feeds, $this->excludeMutedFeeds));
 }
 
 $libopml = new \marienfressinaud\LibOpml\LibOpml(true);