[Bugfix][V1] Fix 'NoneType' object has no attribute 'hash_value' (#11157)

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

vllm/v1/core/kv_cache_manager.py

@@ -164,6 +164,7 @@ class KVCacheManager:
 
         new_full_blocks = req_blocks[
             num_computed_full_blocks:num_full_blocks_after_append]
+        if new_full_blocks:
             self._cache_full_blocks(
                 request=request,
                 blk_start_idx=num_computed_full_blocks,
@@ -375,8 +376,13 @@ class KVCacheManager:
             prev_block: The previous block in the chain.
         """
         # Update the new blocks with the block hashes through the chain.
-        prev_block_hash_value = (prev_block.block_hash.hash_value
-                                 if prev_block is not None else None)
+        prev_block_hash_value = None
+        if prev_block is not None:
+            # Previous block must have a block hash because it must be
+            # a full, cached block.
+            assert prev_block.block_hash is not None
+            prev_block_hash_value = prev_block.block_hash.hash_value
+
         for i, blk in enumerate(full_blocks):
             blk_idx = blk_start_idx + i