gh-36127: Bandaid for polynomial evaluation
There was a problem when evaluating polynomials: the following was
failing
```
sage: x,y=polygens(QQ,'x,y')
sage: t=PolynomialRing(x.parent(),'t').gen()
sage: F=x*y*t
sage: F(x=1)
y*t
```
I propose a fix that hopefully does not break anything else.
We look at a non-zero coefficient to try to guess the correct new base
ring, instead of looking at the constant term.
This does not fix the following inconsistent behaviour:
```
sage: x.parent().zero()(x=1)
0
sage: parent(_)
Rational Field
sage: x.parent().one()(x=1)
1
sage: parent(_)
Multivariate Polynomial Ring in x, y over Rational Field
```
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/36127
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Marc Mezzarobba
