Missing columns when download data from dashboard (when columns have null values)

### Describe the bug

When I download a table from the dashboard, only 35 columns are included. However, when downloading directly from the question, all columns are present in the file.

### To Reproduce

1. Go to a dashboard with one table
2. Click on 'download results'
3. XLSX formated

### Expected behavior

_No response_

### Logs

_No response_

### Information about your Metabase installation

```JSON
{
"browser-info": {
"language": "pt-BR",
"platform": "MacIntel",
"userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36",
"vendor": "Google Inc."
},
"metabase-info": {
"databases": [
"postgres",
"mysql",
"bigquery-cloud-sdk"
],
"run-mode": "prod",
"plan-alias": "pro-self-hosted-yearly",
"version": {
"date": "2024-12-21",
"tag": "v1.52.4",
"hash": "2f8c560"
},
"settings": {
"report-timezone": "America/Sao_Paulo"
},
"hosting-env": "unknown",
"application-database": "postgres",
"application-database-details": {
"database": {
"name": "PostgreSQL",
"version": "15.7"
},
"jdbc-driver": {
"name": "PostgreSQL JDBC Driver",
"version": "42.7.3"
}
}
},
"system-info": {
"file.encoding": "UTF-8",
"java.runtime.name": "OpenJDK Runtime Environment",
"java.runtime.version": "21.0.5+11-LTS",
"java.vendor": "Eclipse Adoptium",
"java.vendor.url": "https://adoptium.net/",
"java.version": "21.0.5",
"java.vm.name": "OpenJDK 64-Bit Server VM",
"java.vm.version": "21.0.5+11-LTS",
"os.name": "Linux",
"os.version": "6.1.90+",
"user.language": "en",
"user.timezone": "GMT"
}
}
```

### Severity

P1

### Additional context

_No response_

Start by reproducing the dashboard's “download results” XLSX export with columns containing null values, then compare it with the direct question export. Trace the dashboard download path and identify where the column set diverges; done means the dashboard file includes all columns and a regression test covers the case.

Written by the indexing model from the issue text.