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

[michaelnebel]

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.