fix: support empty val dataset

[shinytang6]

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What does this PR do?

We have same requirements as https://github.com/volcengine/verl/issues/1676 This PR support user training with empty val dataset

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• Issue Number: Fixes issue # or discussion # if any.
• Training: [Note which backend this PR will affect: FSDP, Megatron, both, or none]
• Inference: [Note which backend this PR will affect: vLLM, SGLang, both, or none]

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[shinytang6]

@vermouth1992 pls help take a look, thx