Update lead source only if empty in SFDC

Hi all,

I am working on creating some global forms in Marketo based on the form types (Contact, Demo, Learn More, etc). I created the first one for a default Contact form, with a hidden lead source as "Web Form". But if the lead source in SFDC is not empty, I don't want Web Form source to override the existing one. I want Web Form synced to SDFC only if the lead source field is empty. Is there anything I can do within the form building to set this logic? I think I could do something like this if I set campaigns, but it is that extra step I have to do for each form type. What is the best practice for this scenario? Thanks!