Don't call sylvester_matrix with zero polynomials

[tornaria]

📚 Description

From #35441 (comment):

$ 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,database_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,polytopes_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,sage_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.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• There are existing tests covering the changes.

[tornaria]

Lint failures are exclusively due to #35418.

[tornaria]

Rebased to 10.0.beta8

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: fc3735d

[tscrim]

LGTM.