text: Title: Question - how do I mark fields as minLength 0 and nullable?
  Body: I have an app using DRF 3.8.2, Django 1.11.16, and drf-yasg 1.10.2.   Schema generation works, but I tried to add automated test cases to verify that the response matches the generated schema.   
  
  I have a CharField on a ModelSerializer that is for a model field that has null=True, blank=True.   Despite this, drf-yasg appears to be generating a minLength: 1 requirement.   Oracle makes no difference between NULL and empty string for VARCHAR2 and NVARCHAR2.   DRF returns these fields as '', which I can change, but is there an easier to control this than a "NullableCharField" subclass?
  
  