Skip dead connections in IADS network

The "no skynet usable units" exception would get triggered when an IADS-node would form its connections. If one of those connections contained an IADS unit that was dead, the exception would get triggered since we weren't skipping "dead connections".

changelog.md

@@ -11,6 +11,7 @@
 
 ## Fixes
 * **[UI/UX]** A-10A flights can be edited again.
+* **[Mission Generation]** IADS bug sometimes triggering "no skynet usable units" error during mission generation
 
 
 # Retribution v1.3.1

game/theater/iadsnetwork/iadsnetwork.py

@@ -130,6 +130,8 @@ class IadsNetwork:
             # but if it does, we want to know because it's supposed to be impossible afaict
             skynet_node = SkynetNode.from_group(node.group)
             for connection in node.connections.values():
+                if not any([x.alive for x in connection.units]):
+                    continue
                 if connection.ground_object.is_friendly(
                     skynet_node.player
                 ) and not game.iads_considerate_culling(connection.ground_object):