fix: check if key is set

Co-authored-by: Côme Chilliet <91878298+come-nc@users.noreply.github.com>
Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

lib/private/App/AppStore/Fetcher/AppFetcher.php

@@ -53,7 +53,7 @@ protected function fetch($ETag, $content, $allowUnstable = false) {
 		/** @var mixed[] $response */
 		$response = parent::fetch($ETag, $content);
 
-		if (empty($response) || $response['data'] === null) {
+		if (!isset($response['data']) || $response['data'] === null) {
 			$this->logger->warning('Response from appstore is invalid, apps could not be retrieved. Try again later.', ['app' => 'appstoreFetcher']);
 			return [];
 		}