Add support for yaml nulls

[david-waltermire]

Committer Notes

Added support for YAML null values: ~ or null.

This issue was raised by usnistgov/oscal-cli#84.

Resolves #157.

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[aj-stein-nist]

I am testing locally and it looks good so far, I will approve and take responsibility for the lack of coverage since I am approving.