fix: fix previoius edge cases such as 0 (#6379)

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Author: MichaelLi65535

libs/langgraph/langgraph/runtime.py

@@ -111,7 +111,7 @@ def merge(self, other: Runtime[ContextT]) -> Runtime[ContextT]:
             stream_writer=other.stream_writer
             if other.stream_writer is not _no_op_stream_writer
             else self.stream_writer,
-            previous=other.previous or self.previous,
+            previous=self.previous if other.previous is None else other.previous,
         )
 
     def override(