[BUG] Pydantic Field nullable=True changes generated schema and Python client

Description

When I generate a schema from

class Class(BaseModel):
  field: Optional[PrefixField] = Field(default=None, nullable=True)

the schema contains

"Class": {
  "properties": {
    "field": {
      "allOf": [
        {
          "$ref": "#/components/schemas/PrefixField"
        }
      ],
    "nullable": true,
}

instead of the desired

"Class": {
  "properties": {
    "field": {
      "$ref": "#/components/schemas/PrefixField"
    },
    "nullable": true,
}

The Python client contains extra files corresponding to Field.md in addition to the desired PrefixField.md, and Class is linked to Field instead of the desired PrefixField.

I get the desired schema and Python client files without nullable=True, but then the field isn't allowed to be set to None.

openapi-generator version

v6.0.1

Reproduce the issue from the shown Pydantic Class model using openapi-generator v6.0.1, then trace schema generation and Python client generation for the nullable field. Compare the output with and without nullable=True. Done means the schema keeps the PrefixField reference without an unwanted Field model, and the client links Class to PrefixField while still allowing None.