[BUG] [typescript-axios] oneOf leads to NULL_SCHEMA_ERR while generating interface

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Using oneOf in the schema and generating code using typescript-axios generator leads to it injecting NULL_SCHEMA_ERR in the interface declaration.

Moreover, it sends a warn log message:

[[rootLedger] assets/openapi/rootledger.yaml] [main] WARN  o.o.codegen.DefaultCodegen - Null schema found. Default type to `NULL_SCHEMA_ERR`

openapi-generator version

7.8.0 stable

OpenAPI declaration file content or url

LedgerFilterBody:
      type: object
      oneOf:
        - required:
            - ledgerId
          properties:
            ledgerId:
              type: string
              description: Filter by ledger ID.
              example: "10120323"
        - required:
            - ledgerName
          properties:
            ledgerName:
              type: string
              description: Filter by ledger name.
              example: "myledger"

Generation Details

{
  "$schema": "./node_modules/@openapitools/openapi-generator-cli/config.schema.json",
  "spaces": 2,
  "generator-cli": {
    "version": "7.8.0",
    "generators": {
      "rootLedger": {
        "generatorName": "typescript-axios",
        "inputSpec": "assets/openapi/rootledger.yaml",
        "output": "src/gen/rootledger",
        "additionalProperties": {
          "supportsES6": "true",
          "withInterfaces": true
        }
      }
    }
  }
}

Generates a problematic interface:

/**
 * 
 * @export
 * @interface LedgerFilterBody
 */
export interface LedgerFilterBody {
    [key: string]: NULL_SCHEMA_ERR; // -- why????

}

This leads to a build error because NULL_SCHEMA_ERR isn't defined.

Steps to reproduce

1. Create a schema file containing LedgerFilterBody (can probably copy-paste)
2. Try to generate code using above config.

Related issues/PRs

Found a few, but none of them lead to NULL_SCHEMA_ERR

Hint

If I restructure my schema like below, it starts generating code fine:

LedgerFilterBody:
  type: object
  properties:
    ledgerId:
      type: string
      nullable: true
      description: Filter by ledger ID.
    ledgerName:
      type: string
      nullable: true
      description: Filter by ledger name.
  oneOf:
    - required: [ledgerId]
    - required: [ledgerName]

/**
 * 
 * @export
 * @interface LedgerFilterBody
 */
export interface LedgerFilterBody {
    /**
     * Filter by ledger ID.
     * @type {string}
     * @memberof LedgerFilterBody
     */
    'ledgerId'?: string | null;
    /**
     * Filter by ledger name.
     * @type {string}
     * @memberof LedgerFilterBody
     */
    'ledgerName'?: string | null;
}

Use the supplied LedgerFilterBody schema and typescript-axios configuration as the reproduction starting point; inspect the generator path that handles oneOf schemas and emits interfaces. Done means generation no longer emits NULL_SCHEMA_ERR for this valid schema and the resulting TypeScript interface builds.