dbutil/queryhelper: return zero-value if no rows found

Without this commit, the QueryOne function would return whatever the
(DataStruct[T]).Scan function returns as the value in the sql.ErrNoRows
error case. In some cases, the value returned  might be an initialized
value. For example, if the following pattern is followed:

	func (s *MyStruct) Scan(row dbutil.Scannable) (*MyStruct, error) {
		err := row.Scan(&s.A, &s.B, &s.C)
		return s, err
	}

Note that even in the error case, the "s" variable will be initialized.

The alternative would be to force the (DataStruct[T]).Scan function to
return nil if there is an error, but that forces unnecessary verbosity
onto the implementer of the interface since they would have to do
something like:

	func (s *MyStruct) Scan(row dbutil.Scannable) (*MyStruct, error) {
		err := row.Scan(&s.A, &s.B, &s.C)
		if err != nil {
			return nil, err
		}
		return s, nil
	}

Signed-off-by: Sumner Evans <[email protected]>

dbutil/queryhelper.go

@@ -117,7 +117,7 @@ func (qh *QueryHelper[T]) scanNew(row Scannable) (T, error) {
 func (qh *QueryHelper[T]) QueryOne(ctx context.Context, query string, args ...any) (val T, err error) {
 	val, err = qh.scanNew(qh.db.QueryRow(ctx, query, args...))
 	if errors.Is(err, sql.ErrNoRows) {
-		err = nil
+		return *new(T), nil
 	}
 	return val, err
 }