Add blank_nullable option to Model Schema

[tomatod]

Summary

Added the blank_nullable option to ModelSchema. The option specifies whether the original Django field's blank option is used to decide nullability.

This is related to the issue #1446. I withdrew the similar PR #1447 once but I thought it was appropriate, so I make this PR again.

By default, this option is True and doesn't change the current behavior considering the compatibility.

Background

Currently, the ModelSchema field generated from a Django field with blank=True is treated as nullable. In this case, the ModelSchema field accept null but Django Model field with null=False may not accept it.

This issue primarily occurs with string-based fields because they have the pattern of blank=True and null=False as one of the general patterns. The pattern is mentioned at null and blank sections in Django Model field Reference.

The null option of Django field is purely database-related according to the above reference. JSON can explicitly express null unlike the action with application/x-www-form-urlencoded content-type. I think as an option it would be good if ModelSchema could simply look only at the original Django field's null option because ModelSchema is literally generated from Model.

Other implementation

I thought of other implementations as bellow.

• An similar option only for string-based field. But, the issue occurs not only with string-based field and this implementation will be a little complicated, so I think the option for all kinds of field is better.
• An change that can get to treat the Django blank option as required property of OpenAPI schema. I think the pattern of blank=True and null=False can be interpreted as "null cannot be used, but unspecifying is OK". Currently, nullable and required property of OpenAPI schema by django-ninja seem to be linked, meaning a field is required when a field is not nullable. But the behavior depends on Pydantic's base_model.model_json_schema() and changing it seems to be complicated.
• An option to set default string to string-based fields. I think this is also limited and not appropriate.

About DRF

In Django Rest Framework, ModelSerializer convert blank and null option on a field of its original Django Model to allow_blank which decides to allow "" or not and allow_null which decides to allow null on its Serializer field.

So I think the users migrating their projects from DRF may also set blank=True and null=False to Django Model and have the same issue.