Enabled nullable reference types in the HotChocolate.Types project

[glen-84]

Summary of the changes (Less than 80 chars)

• Enabled nullable reference types in the HotChocolate.Types project.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (87c992c) to head (1f00c34).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #8413   +/-   ##
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