Generator fails on OAuth2PasswordRequestForm contents

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

首先使用連結的 administration.json OpenAPI 規範重現產生器驗證錯誤，重點關注 OAuth2PasswordRequestForm 欄位及其 nullable schema。當產生器能在沒有驗證錯誤的情況下接受此規範並產生有效的用戶端 schema 時，即表示完成。