CreateBlobStream with bmWrite

[NickNevzorov]

CreateBlobStream with bWrite need to create with clear data

For example standart Delphi code

constructor TClientBlobStream.Create(Field: TBlobField; Mode: TBlobStreamMode);
begin
  FField := Field;
  FFieldNo := FField.FieldNo;
  FDataSet := FField.DataSet as TCustomClientDataSet;
  if Mode <> bmRead then
  begin
    if FField.ReadOnly then
      DatabaseErrorFmt(SFieldReadOnly, [FField.DisplayName], FDataSet);
    if not (FDataSet.State in [dsEdit, dsInsert, dsNewValue]) then
      DatabaseError(SNotEditing, FDataSet);
  end;
  if not FDataSet.GetActiveRecBuf(FBuffer) then Exit;
  if Mode = bmWrite then Truncate
  else ReadBlobData;
end;