Check if selected backend is None in get_attn_backend_cls() (#12975)

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

vllm/platforms/cpu.py

@@ -35,7 +35,7 @@ class CpuPlatform(Platform):
                              dtype: torch.dtype, kv_cache_dtype: Optional[str],
                              block_size: int, use_v1: bool,
                              use_mla: bool) -> str:
-        if selected_backend != _Backend.TORCH_SDPA:
+        if selected_backend and selected_backend != _Backend.TORCH_SDPA:
             logger.info("Cannot use %s backend on CPU.", selected_backend)
         logger.info("Using Torch SDPA backend.")
         return "vllm.attention.backends.torch_sdpa.TorchSDPABackend"