OpenAPI properties may be nullable & required. required only indicates that a key must be present in an object to be valid, it makes no restriction on the type of the value corresponding to that key. openapi-to-graphql assumes that required OpenAPI properties are non-nullable:

To Reproduce
Use openapi-to-graphql on a spec with type with a required field that is also nullable. Queries for that type which return a null for that nullable field cause unexpected validation errors.

Expected behavior
No validation errors when returning nulls for explicitly nullable fields.