You could use the following code in a Transform node (it assumes your input field and output field are both string data types as per your screenshot and defaults to using 1st January 1900 if the first record has a Null value):
#### ConfigureFields property script
out1 += in1
out1.test = str
#### ProcessRecords property script
if node.firstExec:
previousValue = '1900-01-01'
if fields.Column1 is not Null:
out1.test = fields.Column1
previousValue = fields.Column1
else:
out1.test = previousValue