### What this PR does / why we need it?
In vLLM-Omni, there exists the empty `ModelConfig`. We need to add a
check before accessing the sub-field of model_config.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Will checked by CI.
- vLLM version: v0.14.0
- vLLM main:
d68209402d
Signed-off-by: gcanlin <canlinguosdu@gmail.com>