[BugFix] add more `is not None` check in VllmConfig.__post_init__ (#12138)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>

vllm/config.py

@@ -3174,7 +3174,8 @@ class VllmConfig:
 
         if self.compilation_config is None:
             self.compilation_config = CompilationConfig()
-        if envs.VLLM_USE_V1 and not self.model_config.enforce_eager:
+        if envs.VLLM_USE_V1 and self.model_config is not None and \
+            not self.model_config.enforce_eager:
             # NOTE(woosuk): Currently, we use inductor because the piecewise
             # CUDA graphs do not work properly with the custom CUDA kernels.
             # FIXME(woosuk): Disable inductor to reduce the compilation time