fix: Use truthy check for graph_key in __get_track_data

Change `if self._graph_key is not None` to `if self._graph_key` so
that empty string is treated as "not set", matching the truthy check
already used in resumption_token. Prevents round-trip data loss where
a tracker with graph_key="" emits graphKey in events but omits it
from the token.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsonbailey

packages/sdk/server-ai/src/ldai/tracker.py

@@ -189,7 +189,7 @@ def __get_track_data(self) -> dict:
         }
         if self._variation_key:
             data["variationKey"] = self._variation_key
-        if self._graph_key is not None:
+        if self._graph_key:
             data['graphKey'] = self._graph_key
         return data