Generator fails on OAuth2PasswordRequestForm contents

首先使用链接的 administration.json OpenAPI 规范重现生成器验证错误，重点关注 OAuth2PasswordRequestForm 字段及其 nullable schema。当生成器能够在没有验证错误的情况下接受此规范并生成有效的客户端 schema 时，即表示完成。

Describe the bug
The generator reports validation errors on regarding the OAuth2PasswordRequestForm type:

components.schemas.Body_login_auth_login_post.Reference.$ref
  Field required [type=missing, input_value={'properties': {'grant_ty..._login_auth_login_post'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.grant_type.Reference.$ref
  Field required [type=missing, input_value={'anyOf': [{'type': 'stri..., 'title': 'Grant Type'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.grant_type.Schema.anyOf.1.Reference.$ref
  Field required [type=missing, input_value={'type': 'null'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.grant_type.Schema.anyOf.1.Schema.type
  Input should be 'string','number','integer','boolean','array' or 'object' [type=enum, input_value='null', input_type=str]
components.schemas.Body_login_auth_login_post.Schema.properties.client_id.Reference.$ref
  Field required [type=missing, input_value={'anyOf': [{'type': 'stri...], 'title': 'Client Id'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.client_id.Schema.anyOf.1.Reference.$ref
  Field required [type=missing, input_value={'type': 'null'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.client_id.Schema.anyOf.1.Schema.type
  Input should be 'string','number','integer','boolean','array' or 'object' [type=enum, input_value='null', input_type=str]
components.schemas.Body_login_auth_login_post.Schema.properties.client_secret.Reference.$ref
  Field required [type=missing, input_value={'anyOf': [{'type': 'stri...title': 'Client Secret'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.client_secret.Schema.anyOf.1.Reference.$ref
  Field required [type=missing, input_value={'type': 'null'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.3/v/missing
components.schemas.Body_login_auth_login_post.Schema.properties.client_secret.Schema.anyOf.1.Schema.type
  Input should be 'string','number','integer','boolean','array' or 'object' [type=enum, input_value='null', input_type=str]

OpenAPI Spec File
Example File: https://raw.githubusercontent.com/stustapay/stustapay/2812c74c622b11c2aa95d0f59933815c0daca4ed/api/administration.json

Desktop (please complete the following information):

• OS: Fedora Linux 38
• Python Version: 3.11.4
• openapi-python-client version: 0.15.1

Additional context
Probably because of FastAPI 3.1.0 https://github.com/openapi-generators/openapi-python-client/discussions/818