## Code Analysis

### The regression test defines an empty result for the table interface
`testing/go/dolt_tables_test.go:1572-1584`
```go
{
	Query:           `SELECT to_id FROM dolt_commit_diff_bug6 WHERE to_commit = (SELECT commit_hash FROM dolt.log) AND from_commit = dolt_hashof('HEAD~');`,
	ExpectedErr:     "more than",
	ExpectedErrCode: "21000",
},
{
	Query:           `SELECT to_id FROM DOLT_DIFF((SELECT commit_hash FROM dolt.log), 'HEAD', 'bug6');`,
	ExpectedErr:     "more than",
	ExpectedErrCode: "21000",
},
{
	Query:    `SELECT to_id FROM dolt_commit_diff_bug6 WHERE to_commit = (SELECT commit_hash FROM dolt.log WHERE 1 = 0) AND from_commit = dolt_hashof('HEAD~');`,
	Expected: []sql.Row{},
},
```
The local regression fixture treats a zero-row commit selection as an empty result for the table form, while separately checking that multi-row selections produce the PostgreSQL cardinality error. The local adapter in `server/tables/dtables/diff.go:23-43` only defines the diff table schema and table name; it contains no `DOLT_DIFF` function argument conversion. The dependency revisions in `go.mod:11-14` provide the dependency-backed diff and scalar evaluation path.

### Result
Source analysis and the captured SQL matrix support the reported defect: the zero-row table selection returns no rows, but the analogous `DOLT_DIFF` function selection converts the empty scalar result into an internal error instead of an empty diff result.

### Observed execution

#### fixture and successful controls
```sql
DROP TABLE IF EXISTS bug6;
CREATE TABLE bug6 (id integer PRIMARY KEY, v text);
INSERT INTO bug6 VALUES (1, 'a');
SELECT dolt_add('-A');
SELECT dolt_commit('--all', '--message', 'base', '--author', 'A <a@example.com>');
UPDATE bug6 SET v = 'b' WHERE id = 1;
SELECT dolt_add('-A');
SELECT dolt_commit('--all', '--message', 'change', '--author', 'A <a@example.com>');
```
```text
SELECT 'ONE_TABLE' AS case_name, to_id FROM dolt_commit_diff_bug6 WHERE to_commit = (SELECT commit_hash FROM dolt.log ORDER BY date DESC LIMIT 1) AND from_commit = (SELECT commit_hash FROM dolt.log ORDER BY date DESC OFFSET 1 LIMIT 1);
 case_name | to_id
-----------+-------
 ONE_TABLE |     1
(1 row)

SELECT 'ONE_FUNCTION' AS case_name, to_id FROM DOLT_DIFF((SELECT commit_hash FROM dolt.log ORDER BY date DESC OFFSET 1 LIMIT 1), (SELECT commit_hash FROM dolt.log ORDER BY date DESC LIMIT 1), 'bug6');
  case_name   | to_id
--------------+-------
 ONE_FUNCTION |     1
(1 row)
```

#### boundary requests and responses
```sql
SELECT 'ZERO_TABLE' AS case_name, to_id FROM dolt_commit_diff_bug6 WHERE to_commit = (SELECT commit_hash FROM dolt.log WHERE 1 = 0) AND from_commit = dolt_hashof('HEAD~');
```
```text
 case_name | to_id
-----------+-------
(0 rows)
```
```sql
SELECT 'ZERO_FUNCTION' AS case_name, to_id FROM DOLT_DIFF((SELECT commit_hash FROM dolt.log WHERE 1 = 0), 'HEAD', 'bug6');
```
```text
ERROR:  XX000: received '<nil>' when expecting commit hash string
```

#### cardinality control
```text
ERROR:  21000: the subquery returned more than 1 row
ERROR:  21000: the subquery returned more than 1 row
```
The two `21000` responses correspond to the multi-row table and function controls; the `XX000` response is the zero-row function case.

# final result: BF-INTEGRATION-1 failed - the table interface returned an empty result for the zero-row commit selection, while DOLT_DIFF returned SQLSTATE XX000 with a nil commit-hash conversion error.
# test context: The fixture used a local committed bug6 table with no stubs, mocks, or bypasses. The browser screenshot is not used because it shows only the PostgreSQL endpoint's ERR_EMPTY_RESPONSE page, not the SQL evidence.