gh-39852: Handle empty matrices over univariate polynomials
    
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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

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- [X] The title is concise and informative.
- [X] The description explains in detail what this PR is about.
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- [X] I have created tests covering the changes.
- [X] I have updated the documentation and checked the documentation
preview.
    
URL: #39852
Reported by: Biffo89
Reviewer(s): Frédéric Chapoton, Vincent Neiger

Author: Release Manager