Command `init_sanitizer` doesn't handle proxy models correctly

[JsseL]

In a situation where we have models like this:

class Company(models.Model):
    field_x = models.CharField()

class SpecializedCompany(Company):
    class Meta:
        proxy = True

The init_sanitizer command produced:

app_company: {}

It should have created:

app_company:
  field_x: null