Example 1:

This fails

$cron_job_multiple = hiera_hash('cron::job::multiple', undef)
  if $cron_job_multiple  {
    create_resources('cron::job::multiple', $cron_job_multiple)
  }

Example 2:

$cron_job_multiple = hiera_hash('cron::job::multiple', undef)
  if defined($cron_job_multiple)  {
    create_resources('cron::job::multiple', $cron_job_multiple)
  }

This works:
$cron_job_multiple = hiera_hash('cron::job::multiple', '')
  if $cron_job_multiple != '' {
    create_resources('cron::job::multiple', $cron_job_multiple)
  }