I have a few cookie values I want to read into a form. This is currently working.
What doesn't work is when the cookie value is empty, it doesn't overwrite the value that was in the field before. Is this normal Marketo behavior? If yes, I could modify the cookie values to have a NULL instead of an empty value.
Yes, this Marketo does not override field values with empty values from forms.
Yes, you can set you cookies to get a value and use it. Just do not use NULL as it's a word that has a specific meaning and you might have you fields reading "NULL" (test it though )
I prefer to se the cookies to <empty> and if necessary have a smart campaign is Marketo that clean the fields that contain "<empty>".