Official comment
Use the Condition "Equal To" and leave the Value box empty to indicate that the field value is blank, as shown here:

Comment actions
I have to check and if the field is blank, add an action. Not sure how to indicate a blank field. Do I use "null" or something like that?
Use the Condition "Equal To" and leave the Value box empty to indicate that the field value is blank, as shown here:

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Does this apply as well when creating a rule on a field being 'not empty' as well?
Field: Email
Condition: Not equal to
Value 'empty'
Yes, that should work as well.