[Bugfix] Add the check for a null VllmConfig (#4749)

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

In vllm-omni, we create the empty `VllmConfig`, which raised the null
error in
[`vllm-ascend/vllm_ascend/utils.py`](https://github.com/vllm-project/vllm-ascend/blob/a7f91079b8576a846f671c9e6923805e74e35c87/vllm_ascend/utils.py#L833).
More details are
[here](vllm-project/vllm-omni#208).

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

### How was this patch tested?


- vLLM version: v0.12.0
- vLLM main:
vllm-project/vllm@ad32e3e

Signed-off-by: gcanlin <canlinguosdu@gmail.com>

Author: gcanlin

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 and vllm_config.model_config:
+    if _IS_VL_MODEL is None and vllm_config 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