Primary key field should not be nullable

Description

If a value is a primary key, it should not be optional(nullable). For example, there is a Model called Foo, whose primary key is a UUIDField called id. Submitting a form using FooIn will use exclude_readonly=True, so the id field will not be included. And for output FooOut id cannot be nullable because it is the primary key. This PR fixes this problem.

Before:

After:

Motivation and Context

My projects use OpenAPI documentation auto-generated as a TypeScript client. It helps a lot to annotate the types correctly.

How Has This Been Tested?

This is a minor change, and it passes all existing tests. I tested it in a few of my live projects and it works fine.

Checklist:

[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[x] I have added the changelog accordingly.
[x] I have read the CONTRIBUTING document.
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.

Totals
Change from base Build 11887821450:	0.001%
Covered Lines:	6224
Relevant Lines:	6857

tortoise / tortoise-orm