Fix python example list

[therve]

This fixes python list examples on models by not generating the example when we have model objects, and adding the missing closing character when we don't.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
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[therve]

@taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether 🙇

[spacether]

Why are we not seeing a sample change?
Could you add a test sample which uses this code path?

[therve]

I added a test sample, and it made me realize it didn't fix it the way I expected it to be fixed. I changed the code, let me know if it makes sense.

[spacether]

@therve could you rebase on master?
Those jaxrs CI errors are odd and should not be happening.

[spacether]

circleci dart issue is a know issue and is not caused by this pr

[spacether]

Thank you for your PR and for adding the tests. This looks good!
CI errors are unrelated to this PR