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

Beginne damit, die Validierungsfehler des Generators mit der verknüpften administration.json OpenAPI-Spezifikation zu reproduzieren, wobei der Fokus auf den Feldern von OAuth2PasswordRequestForm und deren nullable Schemas liegt. Als abgeschlossen gilt die Aufgabe, wenn der Generator diese Spezifikation ohne Validierungsfehler akzeptiert und ein gültiges Client-Schema erzeugt.