In most cases, an empty field evaluates as though it contains a zero.
The Average aggregate function and Average summary field do not "count" empty fields in the resulting Mean. 4, 0, 2 has a mean of 3. 4, blank , 2 has a mean of 3.
If all the fields referenced in a calculation field are empty and the "do not evaluate if all referenced fields are empty" check box is selected, the calculation does not evaluate and it returns a null (empty) result.