C#: Make Nullable type a ConstructedType and VoidType a ValueType.

[michaelnebel]

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Pull Request checklist

All query authors

• [ ] A change note is added if necessary. See the documentation in this repository.
• [ ] All new queries have appropriate .qhelp. See the documentation in this repository.
• [ ] QL tests are added if necessary. See Testing custom queries in the GitHub documentation.
• [ ] New and changed queries have correct query metadata. See the documentation in this repository.

Internal query authors only

• [ ] Autofixes generated based on these changes are valid, only needed if this PR makes significant changes to .ql, .qll, or .qhelp files. See the documentation (internal access required).
• [ ] Changes are validated at scale (internal access required).
• [ ] Adding a new query? Consider also adding the query to autofix.