fix: Omit variationKey from track data when empty

Match the resumption token behavior: only include variationKey in the
track data dict when it has a non-empty value.

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

Author: jsonbailey

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

@@ -180,12 +180,13 @@ def __get_track_data(self) -> dict:
         """
         data = {
             "runId": self._run_id,
-            "variationKey": self._variation_key,
             "configKey": self._config_key,
             "version": self._version,
             "modelName": self._model_name,
             "providerName": self._provider_name,
         }
+        if self._variation_key:
+            data["variationKey"] = self._variation_key
         if self._graph_key is not None:
             data['graphKey'] = self._graph_key
         return data