Can't skip missing fields in save data action

I want to have a save data action where only the fields that have been filled in will be updated. So naturally in the override I put
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field:{data:field}/-

However on one website I just get "Error:0" (super unhelpful). On another site in the debug under the info section for the save data action I get "field has been skipped" but then it goes ahead and updates the value with an empty string anyway.
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So what I've had to resort to doing is

field:{data:field/(var:read_data#.model.field)}

Hi healyhatman,
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It's a workaround but you could read the saved data record, update the changed fields and then re-save it. That way you do have full control.
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Bob

Yep that's what I'm doing, at the bottom of my post - read the data, and use the existing data as the default value if the field's form data is empty.