Rudder

The detection of empty fields relies only on "isvariable" to detect empty value. However, while it makes sense to use isvariable for empty value when only one empty value is set (due to implementation used in cf-clerk), when we have several values, even when empty, the values are there, but empty.
To have proper detection, we should use both isvariable (for only one value) and strcmp
Reported originaly by Alex Tkachenko in #4342