Normal Form fails with single variable polynomials

[Nathabolin]

Pull request at #38151

Steps To Reproduce

R.=PolynomialRing(QQ)
A=AffineSpace(R)
f=DynamicalSystem_affine(z^2+z+1).homogenize(1)
f.normal_form()

Expected Behavior

Should produce the normal form

Actual Behavior

AttributeError: 'sage.rings.polynomial.polynomial_rational_flint.Polynomial_rational_flint' object has no attribute 'variable'

Additional Information

No response

Environment

- **OS**: Ubuntu
- **Sage Version**: 10.0
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Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide