Handle empty matrices over univariate polynomials

[Biffo89]

Fixes #39587

Adds handling of empty matrices over univariate polynomials, specifically in the following methods:

• degree() (returns -1)
• constant_matrix()
• inverse_series_trunc()
• solve_left_series_trunc()
• row_degrees() ([] if no rows, [-1, ..., -1] if no columns)
• column_degrees() (vice versa)
• is_hermite() (defaults to _is_empty_popov())
• weak_popov_form()
• right_quo_rem()
• _right_quo_rem_reduced()
• minimal_relation_basis()

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[github-actions]

Documentation preview for this PR (built with commit 872c058; changes) is ready! 🎉
This preview will update shortly after each push to this PR.