Allow environment in body agent control to be set to an empty list

   error: Empty list is not a valid value for 'environment' attribute in agent control body

body agent control
{
      # Environment variables of the agent process.
      # The values of environment variables are inherited by child commands
      environment => { @(def.control_agent_environment_vars) };
}
bundle common def
{
    any::

      "control_agent_environment_vars" ->  { "CFE-3925" }
        slist => { },
        if => not( isvariable( "control_agent_environment_vars" ) ),
        comment => concat( "Environment variables of the agent process. The values",
                           " of environment variables are inherited by child commands.",
                           " This promise sets the default to a list with an empty element since it is illegal to set an empty list. n empty list.",
                           " Use Augments to override this default" );
}