[Fix] Alloc return type error (#7778)

Signed-off-by: Capronir <839972205@qq.com>

python/sglang/srt/disaggregation/utils.py

@@ -74,7 +74,7 @@ class ReqToMetadataIdxAllocator:
     def available_size(self):
         return len(self.free_slots)
 
-    def alloc(self) -> List[int]:
+    def alloc(self) -> Optional[int]:
         if len(self.free_slots) == 0:
             return None