[bugfix] fix quant method validation bug (#4831)

### What this PR does / why we need it?
When `hf_quant_cfg` is not None and `hf_quant_cfg.quant_method == ""`,
func `override_quantization_method` will return None and raise
ValidationError.

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: zzzzwwjj <1183291235@qq.com>

tests/ut/quantization/test_quant_config.py

@@ -66,11 +66,19 @@ class TestAscendQuantConfig(TestBase):
         mock_is_available.return_value = True
         result = AscendQuantConfig.override_quantization_method(None, None)
         self.assertIsNone(result)
+        hf_quant_cfg = {"quant_method": ""}
+        result = AscendQuantConfig.override_quantization_method(
+            hf_quant_cfg, None)
+        self.assertEqual(result, "ascend")
 
         # Test when NPU is not available
         mock_is_available.return_value = False
         result = AscendQuantConfig.override_quantization_method(None, None)
         self.assertIsNone(result)
+        hf_quant_cfg = {"quant_method": ""}
+        result = AscendQuantConfig.override_quantization_method(
+            hf_quant_cfg, None)
+        self.assertIsNone(result)
 
     def test_get_quant_method_for_linear(self):
         mock_config = MagicMock()