Fix broken backward compatibility with Document.empty_copy()

[abey79]

Description

#359 renamed Document.empty_copy() to Document.clone() for coherence with LineCollection. However, empty_copy() should have been left for backward compatibility. This PR restores this call.

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy returns no error
• tests added/updated and pytest succeeds
• documentation added/updated
  • command docstring and option/argument help
  • README.md updated (Feature Overview)
  • CHANGELOG.md updated
  • added new command to reference.rst
  • RTD doc updated and building with no error (make clean && make html in docs/)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #380 (9a47476) into master (8b70225) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #380   +/-   ##
=======================================
  Coverage   92.72%   92.72%           
=======================================
  Files          53       53           
  Lines        4534     4535    +1     
  Branches      636      636           
=======================================
+ Hits         4204     4205    +1     
  Misses        220      220           
  Partials      110      110           

Impacted Files	Coverage Δ
vpype/model.py	92.13% <100.00%> (+0.02%)	⬆️

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