extensionDownloader: Only check server if there's something to update

checkForUpdates() will currently always query the server for updates,
even when passing an empty vardict of installed extensions. We know
there won't be any updates in that case, so avoid a pointless network
request.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1100

js/ui/extensionDownloader.js

@@ -127,6 +127,9 @@ function checkForUpdates() {
         metadatas[uuid] = extension.metadata;
     });
 
+    if (Object.keys(metadatas).length === 0)
+        return; // nothing to update
+
     let versionCheck = global.settings.get_boolean(
         'disable-extension-version-validation');
     let params = {