Here is what I have found out via Unbounce Customer Support:
_"To understand the root of the problem better, let’s explore why you’re getting a blank form submission as a conversion. You’re right to say that when a conversion is logged in CallRail, Unbounce is sending a form submission as a conversion regardless of whether the form is filled out or not and more often than not, it’s a blank form submission. _
So, it appears that there is no direct solution as CallRail has written the integration.
One potential work around is to use Zapier to filter out blank lead submissions before sending them to Salesforce, instead of using the native integration with Salesforce (and Campaign Monitor). The down side of this is that I need to build a Zap for every landing page that I have (which are many), making maintenance very cumbersome and prone to errors. (On a separate, non-technical note, I would have to upgrade my subscription, doubling my monthly costs).
Does @katemcghy or @Stefano have any insights?