[MM][Bugfix] Minor fix for VL model verification (#4384)

### What this PR does / why we need it?

To fix ops test, where `model_config` has been set to `None` and doesn't
has `hf_config` attribute, we have added a check for `model_config` to
guarantee it is not `None_Type`.

- vLLM main:
vllm-project/vllm@2918c1b

Signed-off-by: shen-shanshan <467638484@qq.com>

Author: shen-shanshan

vllm_ascend/utils.py

@@ -810,7 +810,7 @@ def _is_contain_expert(config: Any):
 def is_vl_model(vllm_config: VllmConfig):
     """Checks if the model is a VL model by config"""
     global _IS_VL_MODEL
-    if _IS_VL_MODEL is None:
+    if _IS_VL_MODEL is None and vllm_config.model_config:
         model_configs = vllm_config.model_config.hf_config.to_dict()
         _IS_VL_MODEL = "VL" in model_configs["architectures"][0]
     return _IS_VL_MODEL