Empty observation with and without attributes is shown differently
When the observation value is missing and it doesn't have any attribute attached, it is shown as an empty cell.
When the observation is missing and it has some attributes attached, "-" is shown in the cell.
The behaviour should be consistent.
Solution to implement
In any case when there is no observation value returned in a table cell,
With or without flag or footnote associated to the same coordinates,
Then we display a double dot
.. in the cell.