username is nullable in OpenAPI spec

[jennydaman]

ChRIS_ultron_backEnd/chris_backend/users/serializers.py

Line 10 in 0b21e7f

username = serializers.CharField(min_length=4, max_length=32, required=False,

One serializer is reused for different API operations, reducing static type safety. E.g. response from api/v1/users/{id}/ will always contain username however in the OpenAPI spec, the username field is not required.