fix: crash when a string is passed for the topics parameter of the mercure_publish() function (#2021)

testdata/flush.php

@@ -1,6 +1,5 @@
 <?php
 
-
 require_once __DIR__.'/_executor.php';
 
 return function () {

testdata/mercure-publish.php

@@ -0,0 +1,8 @@
+<?php
+
+require_once __DIR__.'/_executor.php';
+
+return function () {
+	echo "update 1: " . mercure_publish('foo', 'bar', true, 'myid', 'mytype', 10) . PHP_EOL;
+	echo "update 2: " . mercure_publish(['baz', 'bar']) . PHP_EOL;
+};