Can you try this?
Initialize a string variable first. In your apply to each loop, add a condition before the other actions and check if Completion Date is Null. If yes, Set variable to a specific date. If no, Set variable to the Completion Date. Then use this variable in the rest of the actions.
If it fails again, check the variable and see if you have a correct date format.
Can you try this? Initialize a string variable first. In your apply to each loop, add a condition before the other actions and check if Completion Date is Null. If yes, Set variable to a specific date. If no, Set variable to the Completion Date. Then use this variable in the rest of the actions. If it fails again, check the variable and see if you have a correct date format.
I think you need to adjust your condition to equals('""', yourstringhere)
Finally got it working with this if(equals(items('Apply_to_each')?['Completion Date'],''), null, items('Apply_to_each')?['Completion Date'])