message: Fix filter->parse_expr not to use .unwrap()
diff: @@ -161,10 +161,12 @@ fn parse_expr<'a, T: 'static + Debug>(
  ConditionBase::NestedSelect(statement) => {
  let params = fetch_select_params(storage, statement)?;
  let value = select(storage, statement, &params, Some(filter_context))?
  - .map(|c| Row::take_first_value(c.unwrap()))
  + .map(|row| -> Result<_> {
  + row?.take_first_value()
  + .ok_or(FilterError::RowIsEmpty.into())
  + })
  .next()
  - .ok_or(FilterError::RowNotFound)?
  - .ok_or(FilterError::RowIsEmpty)?;
  + .ok_or(FilterError::RowNotFound)??;
  Ok(Some(Parsed::Value(value)))
  }
  