unlockDialog: Don't create AuthDialog just to finish it

If the the unlock dialog gets finished before an auth dialog is
created, the code currently creates one just to tell it to finish.

This commit changes the code to skip creating the auth dialog in
that case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1999>

js/ui/unlockDialog.js

@@ -872,7 +872,11 @@ var UnlockDialog = GObject.registerClass({
     }
 
     finish(onComplete) {
-        this._ensureAuthPrompt();
+        if (!this._authPrompt) {
+            onComplete();
+            return;
+        }
+
         this._authPrompt.finish(onComplete);
     }