Ensure NULL string value is written as quoted string in YAML

[adityapatwardhan]

PR Summary

This pull request introduces a new feature to handle NULL values in YAML serialization by quoting them, along with corresponding updates to tests and serializers. The most important changes include adding a custom event emitter for quoting NULL strings, updating the YAML serializer to use this emitter, and adding tests to validate the new behavior.

YAML Serialization Enhancements:

• Custom Event Emitter for Quoting NULL: Added a new class QuotedNullStringEventEmitter that ensures NULL values are serialized as 'NULL' in YAML. This class overrides the Emit method to handle NULL strings explicitly. (src/YamlWriter/QuotedNullStringEventEmitter.cs)
• Updated YAML Serializer: Modified the YamlUtils class to use the new QuotedNullStringEventEmitter in the YAML serializer configuration. (src/Common/YamlUtils.cs)

Testing Updates:

• New Test for NULL Handling: Added a test case in Export-YamlCommandHelp to verify that NULL values are correctly quoted as 'NULL' in the serialized YAML output. (test/Pester/ExportYamlCommandHelp.Tests.ps1)
• Updated Asset Counts in Tests: Adjusted expected counts in MeasurePlatyPSMarkdown tests to reflect the addition of new test assets. (test/Pester/MeasurePlatyPSMarkdown.Tests.ps1)

PR Context

Fixes #758

[adityapatwardhan]

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[sdwheeler]

LGTM

[adityapatwardhan]

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