Fixed nullability attributes generation on .NET Standard targets

[Sergio0694]

Fixes #4167

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Build error due to unusable attributes when using [INotifyPropertyChanged] on .NET Standard 2.0.

What is the new behavior?

No build errors, attributes are generated as expected when needed.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks Sergio0694 for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌