Preserve character output buffers when SQLGetData returns NULL binary values

Describe the bug

SQLGetData on a NULL binary column writes a character terminator into the caller's output buffer in mssql-odbc, while retail msodbcsql leaves the buffer untouched. Both return SQL_SUCCESS and SQL_NULL_DATA. This predates the binary-to-hex conversion in #530 and should be handled separately from that feature.

Steps to reproduce

1. Connect to a test SQL Server through the ODBC Driver Manager.
2. Execute SELECT CAST(NULL AS VARBINARY(MAX)) and fetch the row.
3. Fill a writable 32-byte buffer with 0x7E.
4. Call SQLGetData for column 1 with SQL_C_CHAR, buffer length 32, and a valid indicator pointer. Repeat on a fresh row with SQL_C_WCHAR.
5. Compare the return code, indicator, and buffer contents against retail msodbcsql. The same difference was measured with BINARY(3) and VARBINARY(8).

Expected behavior

For intended parity with retail msodbcsql, return SQL_SUCCESS and SQL_NULL_DATA without modifying the character output buffer. Confirm the behavior on the CI-pinned retail 18.6.2.1 build before implementing the follow-up; the runtime comparison below used 18.6.1.1.

Actual behavior

For SQL_C_CHAR, mssql-odbc changes the first byte to zero. For SQL_C_WCHAR, it changes the first two bytes to zero. The remaining bytes retain 0x7E. Retail msodbcsql 18.6.1.1 (SQL_DRIVER_VER=18.06.0001) leaves every byte untouched in both cases. Return code 0 and indicator -1 agree between drivers.

Version

PR #530 at 3b882725, with main through 0d431b71 merged before the measurement. The observed NULL-output behavior is pre-existing, not introduced by the hex conversion.

Affected crate

mssql-odbc

Environment

Windows x64, SQL Server 2025 test container, Windows ODBC Driver Manager. Reference DLL product version 18.6.1.1, SQL_DRIVER_VER=18.06.0001. Rust driver reports SQL_DRIVER_VER=18.6.2.1.

Additional context

• Raised in https://github.com/microsoft/mssql-rs/pull/530#discussion_r3984006173.
• Rust: mssql-odbc/src/api/get_data.rs::write_captured_column handles ColumnValues::Null before conversion and calls copy_with_nul for character targets. Audit other NULL delivery branches and bound delivery for consistent behavior before changing anything.
• Reference source: Sql/Ntdbms/sqlncli/odbc/sqlcdata.h, ColDataRetriever::InternalGetColData: fIsNullData sets SQL_NULL_DATA and jumps to Return2, bypassing character termination under Return3.
• BinToCharLiveTest.NullVarbinaryMaxReportsNull intentionally asserts only the shared NULL indicator and runs on both drivers. Keep it unskipped; add a targeted buffer-content parity regression with the follow-up fix.
• This is a deferred implementation gap, not an approved permanent deviation. The repository's deliberate-deviation registry explicitly excludes such gaps.

Start in mssql-odbc/src/api/get_data.rs at write_captured_column, then audit the other NULL-delivery and bound-delivery branches. Confirm the behavior against the CI-pinned retail 18.6.2.1 driver, keep BinToCharLiveTest.NullVarbinaryMaxReportsNull enabled, and add a targeted regression covering unchanged character buffers for SQL_C_CHAR and SQL_C_WCHAR.