Address [NotNull] on int?

[viceroypenguin]

Fixes #123

[codecov]

Codecov Report

Attention: Patch coverage is 93.22034% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ate.Validations.Analyzers/ValidateClassAnalyzer.cs	90.32%	2 Missing and 1 partial ⚠️
....Validations.Shared/Validators/NotNullAttribute.cs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/Common/ITypeSymbolExtensions.cs	96.38% <100.00%> (+0.26%)	⬆️
...AddAdditionalValidationsCodeRefactoringProvider.cs	88.00% <100.00%> (ø)
src/Immediate.Validations.Generators/Models.cs	100.00% <100.00%> (ø)
...alidations.Generators/ValidateTargetTransformer.cs	94.97% <100.00%> (+0.03%)	⬆️
....Validations.Shared/Validators/NotNullAttribute.cs	0.00% <0.00%> (ø)
...ate.Validations.Analyzers/ValidateClassAnalyzer.cs	95.82% <90.32%> (+<0.01%)	⬆️