Fix viewer crash with empty layers

[abey79]

Description

The viewer crashes when presented with empty layers (eg due to crop).

Fixes #338

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy vpype vpype_cli tests 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
  • RTD doc updated and building with no error (make clean && make html in docs/)

[codecov]

Codecov Report

Merging #339 (4497036) into master (2cebb44) will increase coverage by 92.35%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #339       +/-   ##
===========================================
+ Coverage        0   92.35%   +92.35%     
===========================================
  Files           0       50       +50     
  Lines           0     3989     +3989     
  Branches        0      522      +522     
===========================================
+ Hits            0     3684     +3684     
- Misses          0      203      +203     
- Partials        0      102      +102     

Impacted Files	Coverage Δ
tests/test_viewer.py	100.00% <100.00%> (ø)
vpype_viewer/engine.py	96.42% <100.00%> (ø)
vpype/config.py	97.87% <0.00%> (ø)
tests/test_model.py	100.00% <0.00%> (ø)
tests/test_layers.py	100.00% <0.00%> (ø)
vpype_cli/primitives.py	100.00% <0.00%> (ø)
vpype_cli/generators.py	100.00% <0.00%> (ø)
vpype/io.py	78.23% <0.00%> (ø)
vpype/primitives.py	61.40% <0.00%> (ø)
vpype/__init__.py	100.00% <0.00%> (ø)
... and 42 more

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[abey79]

@cbmoore May I ask if this PR fixes the issue? (I'm still struggling with Monterey/M1 issues at the moment)

Use the following command to install from this branch:

pip install -U git+https://github.com/abey79/vpype@fix-empty-layer-crash-338#egg=vpype[all]

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[cbmoore]

@abey79 Sorry it took me so long to get back to you--looks like you already tested and merged but if there's anything else I can do to be helpful. Thanks for addressing this issue so quickly.

[abey79]

@cbmoore No worries. Reporting issues is already very helpful!