Gracefully handle explicit transient=None (#322)

* gracefully handle explicit transient=None re #321

* rephrase

nbclient/client.py

@@ -1100,7 +1100,12 @@ def process_message(
         content = msg["content"]
         self.log.debug("content: %s", content)
 
-        display_id = content.get("transient", {}).get("display_id", None)
+        # while it's tempting to go for a more concise
+        # display_id = content.get("transient", {}).get("display_id", None)
+        # this breaks if transient is explicitly set to None
+        transient = content.get("transient")
+        display_id = transient.get("display_id") if transient else None
+
         if display_id and msg_type in {"execute_result", "display_data", "update_display_data"}:
             self._update_display_id(display_id, msg)