bug #48993 [VarDumper] Fix JS to expand / collapse (nicolas-grekas)

This PR was merged into the 5.4 branch.

Discussion
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[VarDumper] Fix JS to expand / collapse

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #48545
| License       | MIT
| Doc PR        | -

As hinted in facebook/react#25897 (comment)

Commits
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19add97 [VarDumper] Fix JS to expand / collapse

src/Symfony/Component/VarDumper/Dumper/HtmlDumper.php

@@ -167,9 +167,9 @@ protected function getDumpHeader()
     };
 
 refStyle.innerHTML = 'pre.sf-dump .sf-dump-compact, .sf-dump-str-collapse .sf-dump-str-collapse, .sf-dump-str-expand .sf-dump-str-expand { display: none; }';
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 refStyle = doc.createElement('style');
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 
 if (!doc.addEventListener) {
     addEventListener = function (element, eventName, callback) {