Avoid running string functions when domain is null (#258)

Fixes an issue in PHP 8.1 where a request with no `Referer` or `Origin`
header causes a "deprecated" `ErrorException` when `Str::replaceFirst`
attempts to call `strpos` with `null` as the first argument.

src/Http/Middleware/EnsureFrontendRequestsAreStateful.php

@@ -57,6 +57,10 @@ public static function fromFrontend($request)
     {
         $domain = $request->headers->get('referer') ?: $request->headers->get('origin');
 
+        if (is_null($domain)) {
+            return false;
+        }
+
         $domain = Str::replaceFirst('https://', '', $domain);
         $domain = Str::replaceFirst('http://', '', $domain);
         $domain = Str::endsWith($domain, '/') ? $domain : "{$domain}/";