fix: guard against None chat_template in _post_process_chat_template #4684

Job	Run time
pr-gate / check-file-changes	23s
pr-gate / wait-checks / wait	11m 47s
gpu-tests (gpu_megatron, 45, nemo:26.04)	56s
gpu-tests (gpu, 60, pytorch:26.03-py3)	56s
gpu-tests (gpu_trtllm, 30, tensorrt-llm/release:1.3.0rc12)	56s
gpu-pr-required-check	2s
	15m 0s

Provide feedback