Add test cases for serializing empty collection nodes

[fktn-k]

This PR adds some test cases which validate serialization results from empty collection nodes.
The bug fix itself has already been addressed in the PR #456, so the changes in the library sources are only for improvements in readability and coverage.

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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
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[github-actions]

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Coverage data has been uploaded for the commit b7d69c53ecff116fd59b61f213634d5c7ac418fc.
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Name	fkYAML_coverage.pr457.zip
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[fktn-k]

See also: #454