fix(slack): use explicit None-coalesce for DM thread_ts at encode time

Per CLAUDE.md port rule, prefer 'x if x is not None else default' over 'x or default'. Functionally identical here (only falsy value is the intended empty string) and keeps the SlackThreadId(thread_ts: str) guarantee that pyrefly checks.

Author: claude

src/chat_sdk/adapters/slack/adapter.py

@@ -1624,7 +1624,12 @@ def _handle_block_actions(self, payload: dict[str, Any], options: WebhookOptions
 
         thread_id = ""
         if channel and (thread_ts or message_ts):
-            thread_id = self.encode_thread_id(SlackThreadId(channel=channel, thread_ts=thread_ts or ""))
+            thread_id = self.encode_thread_id(
+                SlackThreadId(
+                    channel=channel,
+                    thread_ts=thread_ts if thread_ts is not None else "",
+                )
+            )
 
         is_ephemeral = (payload.get("container") or {}).get("is_ephemeral") is True
         response_url = payload.get("response_url")