I recently signed up for SiteLock and noticed that we are getting some empty submissions from some of the forms on our web site

SiteLock probes your site to determine if fields and forms on your site are vulnerable to attempts by hackers looking to exploit these forms to gain access to your data. (Though they use similar techniques as hackers to test your pages, you do not need to worry; the process is safe). This will result in attempts to submit forms on your website with encoded data.

If you wish to stop receiving these e-mail's or entries, you may want to do some validation on the fields within your form to ensure that data is being submitted in the correct formats before triggering e-mail's or database inputs. Since they insert data that would not likely be valid for any fields on your site, these validation measures should stop you from getting these empty e-mail's or entries. It's also good coding and security practice to make sure your site's visitors are providing the correct data in the expected formats.