[Bugfix][Diffusion] Canonicalize cache_backend=None to "none" (#7041)

Signed-off-by: summer <128961079+zhang-keliang@users.noreply.github.com>
Co-authored-by: Alicia <115451386+congw729@users.noreply.github.com>

tests/diffusion/models/sana_video/test_cache_offload.py

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
-# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
 
 from dataclasses import dataclass
 from types import SimpleNamespace
@@ -155,6 +155,11 @@ def test_sana_video_rejects_unvalidated_cache_backends():
         _validate_cache_offload_parallelism(OmniDiffusionConfig(cache_backend="tea_cache"))
 
 
+def test_sana_video_accepts_none_cache_backend():
+    """An explicit ``cache_backend=None`` means "no cache", same as ``"none"``."""
+    _validate_cache_offload_parallelism(OmniDiffusionConfig(cache_backend=None))
+
+
 def test_sana_video_rejects_cache_dit_with_distributed_layerwise_offload():
     config = OmniDiffusionConfig(cache_backend="cache_dit", enable_distributed_layerwise_offload=True)