use "php code" in page manager access rules to evaluate if current og_group has content of a specific type
Direct answer: To do this with the code you provided, you should use something like the following:

$node = $contexts['argument_entity_id:node_1']-&gt;data;
$args = array($node-&gt;nid‌​);
return empty(views_get_view_result('rbv_view_group_content','panel_pane_3',$args));


Alternative answer: I have an alternative suggestion, and unless you absolutely want to use the Page Manager, it should allow you to solve your problem without any coding.  

So, if you read so far... :-), I would suggest using Views and setting up a path that will 'link' your view to your group node's URL. You are probably already using a view to list your Events. You can set up a page for that view that has a path like the following:

node/%nid/events


You should further configure your page to be rendered as a "Menu Tab", titled "Events".

This Views Page will now be linked to your Group page, and you will be able to handle its behaviour in the case of no results (Events) for that og_group node; for your case to simply hide it.  

To get this all working, you would also need to set up your contextual filters to use the og_group nid.

This is a bit theoretical, but I am not entirely sure of your setup.
I hope it helps you though!