Adds the empirical normalizer in the ONNX model export for RSL-RL `play.py`
script. Earlier, this was not being done, which would cause empirical
normalization to not work in the ONNX model.

- Bug fix (non-breaking change which fixes an issue)

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`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
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- [ ] I have added tests that prove my fix is effective or that my
feature works
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