sql: investigate failures to prepare statement during session migration [no values for placeholder for IMPORT statement]

**Describe the problem**

In our [Splunk SLI dashboard](https://cockroachlabsgcp.splunkcloud.com/en-US/app/search/search?earliest=-7d%40h&latest=now&q=search%20index%3Dcc-app-crdb*%20%22could%20not%20prepare%20statement%20during%20session%20migration%22%20AND%20(NOT%20%22does%20not%20exist%22)%20%7C%20timechart%20span%3D1h%20count%20by%20friendly_id&display.page.search.mode=verbose&dispatch.sample_ratio=1&display.general.type=statistics&display.page.search.tab=events&sid=1710782971.90343&workload_pool=standard_perf) we have seen this error:

```
could not prepare statement during session migration (IMPORT INTO ?warehouse? CSV DATA ($1, $2) WITH OPTIONS ("nullif" = ?'NULL'?)): no value provided for placeholder: $‹×›
```

This issue tracks work to identify the cause and determine if we can handle this better.

Jira issue: CRDB-36808

Epic CRDB-60943