dev/core#1588 Fix regression where empty string is passed to SettingsBag

dev/core#1588 Fix regression where empty string is passed to SettingsBag

We have a scenario where the checkbox is presented but is optional. This is then in 'params' which is passed through to
PropertyBag. The value is equal to '' so it fails to validate when we use it to set the value on the PropertyBag.

I don't think we lose anything meaningful by not setting an empty string and it avoids this error so we should
merge & release as a regression fix IMHO. If we want to revist then we should do that in master

https://lab.civicrm.org/dev/core/issues/1588