Transform script must be added at mapping creation

Per the test below, even if the index does not contain any data, transform script have to be created at mapping creation time:

```
DELETE /transform_test
PUT /transform_test
PUT /transform_test/album/_mapping
{
    "album": {
      "properties": {
        "artist_name": {
          "type": "string"
        },
        "artist_name2": {
          "type": "string"
        }
      }
    }
  
}
GET /transform_test/album/_mapping
PUT /transform_test/album/_mapping
{
    "album": {
      "transform": {
        "script": "ctx._source['artist_name2'] = ctx._source['artist_name']"
      },
      "properties": {
        "artist_name": {
          "type": "string"
        },
        "artist_name2": {
          "type": "string"
        }
      }
    }
  
}
GET /transform_test/album/_mapping
DELETE /transform_test
PUT /transform_test
PUT /transform_test/album/_mapping
{
    "album": {
      "transform": {
        "script": "ctx._source['artist_name2'] = ctx._source['artist_name']"
      },
      "properties": {
        "artist_name": {
          "type": "string"
        },
        "artist_name2": {
          "type": "string"
        }
      }
    }
  
}
GET /transform_test/album/_mapping
```