Fix detection of webfinger endpoint with no 'resource' param

This is necessary to make the overview test pass.
$subject was null so strpos was throwing an exception, probably since php8.

Signed-off-by: Louis Chemineau <louis@chmn.me>

lib/WellKnown/WebfingerHandler.php

@@ -101,7 +101,7 @@ class WebfingerHandler implements IHandler {
 	 * @return IResponse|null
 	 */
 	public function handleWebfinger(IRequestContext $context): ?IResponse {
-		$subject = $context->getHttpRequest()->getParam('resource');
+		$subject = $context->getHttpRequest()->getParam('resource') ?? '';
 		if (strpos($subject, 'acct:') === 0) {
 			$subject = substr($subject, 5);
 		}