I can't see the exact problem without seeing your twbx workbook and the data but it it appears that the issue is not that the data is null but that there no record in the data set that meets the conditions
In that case you need to pad the data with a date scaffold - see the video Solving tough time-based problems with skeleton tables and Tableau Prep - YouTube
Archana Ganeshalingam and Sasha Singh did the presentation at TC18 and it will take you through the steps - they use PREP but you can to the same in tableau proper
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