fix(export/bigquery): fixes schema issue with untyped objects in data contract

[ricardolpd]

## Description:

Data models with no properties (untyped maps) currently create empty records which in turn create invalid bq schemas:

   someJson:
        type: object
        required: false

generate the following

{
        "name": "someJson:",
        "type": "RECORD",
        "mode": "NULLABLE",
        "description": null,
        "fields": []
      },

which causes the following error:

Error: googleapi: Error 400: Field rootObject.someJson is type RECORD but has no schema, invalid
│ 

But it should generate a JSON type instead:

{
    "name": "someJson",
     "type": "JSON",
     "mode": "NULLABLE",
     "description": null
}

This pr does a small fix to allow these types which can happen when having generic JSON maps

• Tests pass
• ruff format
• README.md updated (if relevant)
•  CHANGELOG.md entry added