Use `is not None` check for peer-connect `event`

Matches the explicit `dict.pop(uid, None)` contract one
line above; same semantics as the prior truthy check.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit 0e3e008b0c)

tractor/ipc/_server.py

@@ -398,7 +398,7 @@ async def handle_stream_from_peer(
         uid,
         None,
     )
-    if event:
+    if event is not None:
         con_status_steps += (
             ' -> Waking subactor spawn waiters: '
             f'{event.statistics().tasks_waiting}\n'