[fix] Set Radix tree root node hash to None - Nvidia Dynamo Integration (#9030)

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

python/sglang/srt/mem_cache/radix_cache.py

@@ -495,7 +495,7 @@ class RadixCache(BasePrefixCache):
         # One BlockStored per ``page_size`` chunk.
         if self.enable_kv_cache_events:
             # First chunk links to the last page of the parent node (if any).
-            if node.parent is None:
+            if node.parent is None or node != self.root_node:
                 parent_block_hash = None
             else:
                 last_page_start = (