gh-40436: Remove useless limitations in Drinfeld modules
    
(1) We authorize Drinfeld modules with constant coefficient zero (which
is definitely not discarded by the theory).
(2) We automatically coerce the given coefficients to the field of
fractions; this avoids one step when creating a Drinfeld module.

### 📝 Checklist

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- [x] The title is concise and informative.
- [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.
- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

#40430
    
URL: #40436
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière, Xavier Caruso

Author: Release Manager