Fix #44618: Fetching may rely on uninitialized data

Fix #44618: Fetching may rely on uninitialized data

Unless `SQLGetData()` returns `SQL_SUCCESS` or `SQL_SUCCESS_WITH_INFO`,
the `StrLen_or_IndPtr` output argument is not guaranteed to be properly
set.  Thus we handle retrieval failure other than `SQL_ERROR` by
yielding `false` for those column values and raising a warning.

Closes GH-6281.