Can you avoid overwriting fields when values are NULL/blank?

Is there a way avoid overwriting data when a form is submitted with blank/null values? 

We use a single webform in several different places on our website, and the form fields differ depending on which form the user is filling out, so we hide unused fields.  When the user submits the form, the hidden fields get submitted too, and any existing data on them is wiped out. 

We can't use Marketo's built-in feature to block field updates, because it's not possible to use it to block ONLY the null/blank values.  It retains the original value for the lifetime of the field, which is not our goal.

https://docs.marketo.com/display/public/DOCS/Block+Updates+to+a+Field