Option to exclude Django Field blank option from determination of 'nullable'

[tomatod]

Is your feature request related to a problem? Please describe.
When blank=True is specified at a Djang Field, ModelSchema treats the field as nullable even if null=False is also specified there. So, ModelSchema passes null to the Django model and it raise NOT NULL constraint exception.

class CharFieldVerification(models.Model):
    char_field = models.CharField(max_length=200, blank=True, null=False)

class TextFieldVerification(models.Model):
    text_field = models.TextField(blank=True, null=False)

class DateFieldVerification(models.Model):
    date_field = models.DateField(blank=True, null=False)

class FileFieldVerification(models.Model):
    file_field = models.FileField(blank=True, null=False)

class BooleanFieldVerification(models.Model):
    boolean_field = models.BooleanField(blank=True, null=False)

The combination of blank=True and null=False seems to be common in string-based Django fields, which is mentioned in Django Model field reference

Other related Issue

• [BUG] ModelSchema does not validate Blank fields properly #882

[tomatod]

According to the Django reference, the blank option is mainly for form validation. Django is originally made for form applications and the target content-type is application/x-www-form-urlencoded, so I think Django field has the blank option because the content-type cannot express null explicitly.

Json can express null originally, so I think as an option it would be good if ModelSchema could simply look only at the null option.