fix(api): Allow null associations on non-required fields.

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
#2431

Description of changes:
Don't throw an exception if an association field is null when creating a mutation.

This is because it is valid for the field to be null if it's not required. Codegen does not mark such fields as required in the generated models. If it is required then attempting to submit the mutation with a null value will result in an error response.

How did you test these changes?
Checked that the schema provided in the original issue could now create a Note class without a customer association.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.