gh-35449: Don't call `sylvester_matrix` with zero polynomials
### :books: Description
From
https://github.com/sagemath/sage/pull/35441#issuecomment-1499566769:
```
$ sage -t --random-seed=3462219213258779680603652694803022552
/usr/lib/python3.11/site-
packages/sage/rings/polynomial/multi_polynomial.pyx
too many failed tests, not using stored timings
Running doctests with ID 2023-04-06-17-14-49-7f2db03c.
Running with SAGE_LOCAL='/usr' and SAGE_VENV='/usr'
Using --optional=pip,sage
Features to be detected: 4ti2,benzene,bliss,buckygen,conway_polynomials,
csdp,cvxopt,database_cremona_ellcurve,database_cremona_mini_ellcurve,dat
abase_cubic_hecke,database_jones_numfield,database_knotinfo,dvipng,gfan,
graphviz,imagemagick,jupymake,kenzo,latte_int,lrslib,mcqd,meataxe,msolve
,nauty,palp,pandoc,pdf2svg,pdftocairo,phitigra,plantri,polytopes_db,poly
topes_db_4d,pynormaliz,python_igraph,rubiks,sage.combinat,sage.geometry.
polyhedron,sage.graphs,sage.groups,sage.misc.cython,sage.plot,sage.rings
.number_field,sage.rings.padics,sage.rings.real_double,sage.symbolic,sag
e_numerical_backends_coin,sagemath_doc_html,sphinx,tdlib
Doctesting 1 file.
sage -t --random-seed=3462219213258779680603652694803022552
/usr/lib/python3.11/site-
packages/sage/rings/polynomial/multi_polynomial.pyx
**********************************************************************
File "/usr/lib/python3.11/site-
packages/sage/rings/polynomial/multi_polynomial.pyx", line 1390, in
sage.rings.polynomial.multi_polynomial.MPolynomial.sylvester_matrix
Failed example:
f.sylvester_matrix(g, x).determinant() == f.resultant(g, x)
Exception raised:
Traceback (most recent call last):
File "/usr/lib/python3.11/site-packages/sage/doctest/forker.py",
line 695, in _run
self.compile_and_execute(example, compiler, test.globs)
File "/usr/lib/python3.11/site-packages/sage/doctest/forker.py",
line 1093, in compile_and_execute
exec(compiled, globs)
File "<doctest sage.rings.polynomial.multi_polynomial.MPolynomial.
sylvester_matrix[9]>", line 1, in <module>
f.sylvester_matrix(g, x).determinant() == f.resultant(g, x)
^^^^^^^^^^^^^^^^^^^^^^^^
File "sage/rings/polynomial/multi_polynomial.pyx", line 1474, in
sage.rings.polynomial.multi_polynomial.MPolynomial.sylvester_matrix
(build/cythonized/sage/rings/polynomial/multi_polynomial.c:17071)
raise ValueError("The Sylvester matrix is not defined for zero
polynomials")
ValueError: The Sylvester matrix is not defined for zero polynomials
**********************************************************************
1 item had failures:
1 of 36 in
sage.rings.polynomial.multi_polynomial.MPolynomial.sylvester_matrix
[577 tests, 1 failure, 3.72 s]
----------------------------------------------------------------------
sage -t --random-seed=3462219213258779680603652694803022552
/usr/lib/python3.11/site-
packages/sage/rings/polynomial/multi_polynomial.pyx # 1 doctest failed
----------------------------------------------------------------------
Total time for all tests: 8.8 seconds
cpu time: 3.5 seconds
cumulative wall time: 3.7 seconds
Features detected for doctesting:
```
This is caused by a (random) polynomial that happens to be zero, which
the `sylvester_matrix()` doesn't allow.
This PR fixes this so when `R.random_element(4)` returns 0, it uses `x^2
* y^2` instead.
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] There are existing tests covering the changes.
URL: https://github.com/sagemath/sage/pull/35449
Reported by: Gonzalo Tornaría
Reviewer(s): Travis Scrimshaw
