Stephen, for this specific instance you'd probably want either Form Fillout, or Flow Data Change Value, depending how the leadsource is populated (hidden form field, or change data value in the flow steps).
I like Jonathan's suggestion, but it seems to be more of a temporary fix, where you need to remember to add it each time. If this is something you'll want to protect going forward, this lessens the likelihood that someone will forget to add the "update if empty"