[BugFix] Fix MultiConnector test after HMA changes (#19291)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/v1/core/kv_cache_manager.py

@@ -61,6 +61,10 @@ class KVCacheBlocks:
             if block.block_hash is None
         ]
 
+    def new_empty(self) -> "KVCacheBlocks":
+        """Creates a new KVCacheBlocks instance with no blocks."""
+        return KVCacheBlocks([[] for _ in range(len(self.blocks))])
+
 
 class KVCacheManager: