Update libcachesim/synthetic_reader.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: haochengxia

libcachesim/synthetic_reader.py

@@ -185,8 +185,8 @@ def get_read_pos(self) -> float:
     def get_working_set_size(self) -> Tuple[int, int]:
         """Calculate working set size"""
         wss_obj, wss_byte = 0, 0
-        if self._obj_ids is not None:
-            unique_ids = np.unique(self._obj_ids[:self.current_pos])
+        if self.current_pos > 0:
+            unique_ids = np.unique(self.obj_ids[:self.current_pos])
             wss_obj = len(unique_ids)
             wss_byte = wss_obj * self.obj_size
         return wss_obj, wss_byte