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You can check the whether the date is null using , ISDATE Retruns true if its null
It may depend on how your data is structured and how your worksheet is laid out. My guess would be that you have spots where there are no underlying rows, not just nulls in existing rows. There are ways to overcome this issue, but I want to be sure this is the problem you are having.
Could you post a .twbx to demonstrate this? If your data is private this could be made up data (with a similar structure to yours) or something public like superstore sales that is included with Tableau. Having something to work with makes it easier to help.