You may want to specify behavior within a Formula field for when a field that your referencing, is empty. You can do this by using a Formula field with an IF  condition:

IF({field name}="", "empty", "not empty")

In this formula {field name}  is a reference to another field that you've created and "empty"  and "not empty"  represent the values that the Formula field should equal, in either situation.

To learn more about Formula fields, see this article.

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