[v1][KVCacheManager] Add a special KVCacheNullBlock class

[heheda12345]

Add a special KVCacheNullBlock class with ref_cnt always equals 1, so that we can simplify the check that a block is free from block.ref_cnt == 0 and block != self.null_block to block.ref_cnt == 0.

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[markmc]

To fix the entrypoints test failure, rebase to pick up #18543

[WoosukKwon]

As we discussed offline, I'd prefer adding is_null attribute to KVCacheBlock and explicitly checking it.
Maybe we can add an assert to the free method, so that we know when the null block is freed.

[heheda12345]

redo in #19031