What happened?

When enabling .withAutoSchemaUpdate(true) and .ignoreUnknownValues() with BigQueryIO.write() method, unexpected behavior happens causing records with nested arrays to fail being written to BQ. An empty record is added into the array, causing schema validation issues. This behavior does not occur without these two settings enabled.

Expected behavior:
Records should be mapped without additional empty objects being added to array

Actual Behavior:
Records in arrays are unexpectedly populated with an empty record, resulting in schema validation failures during write operations.

Steps to reproduce:

1. Use BigQueryIO.write() with: .withAutoSchemaUpdate(true) and .ignoreUnknownValues() (Beam version is 2.59.0)
2. Write records containing array data with the schema mentioned above
3. Observe failures due to an empty record being inserted into arrays.

Example big querry nested record field:

{
  "name": "external_ids",
  "mode": "REPEATED",
  "type": "RECORD",
  "fields": [
    {
      "name": "id",
      "mode": "REQUIRED",
      "type": "STRING"
    },
    {
      "name": "name",
      "mode": "REQUIRED",
      "type": "STRING"
    }
  ]
}

This is what response would look like:

(...)
"errorMessage": "Field value of id cannot be empty. on field external_ids.",
(...)
  "stringifiedData": {
    "external_ids": [
      {
        "id": "fd7da837-13cb-4b67-8ced-220a27130e34",
        "name": "some_custom_id"
      },
      {}
    ],
  }
(...)

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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