fix(AppStore\Fetcher): Ensure `get` returns an array

When fetch fails and the `data` prop contains null (e.g. `json_decode` failed),
then we should return an empty array instead of null.

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

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

@@ -181,7 +181,7 @@ abstract class Fetcher {
 		try {
 			$responseJson = $this->fetch($ETag, $content, $allowUnstable);
 
-			if (empty($responseJson)) {
+			if (empty($responseJson) || empty($responseJson['data'])) {
 				return [];
 			}