hiera hashes with no key-value pairs for feeding into create_resources

gistfile1.yml

# How does one define a hash with a name but no keys/values?
---
site_repos_ppa:
  'ppa:git-core/ppa':
  'ppa:mercurial-ppa/releases':

gistfile2.pp

# calls to apt::ppa usually look like this:

apt::ppa { 'ppa:git-core/ppa': }
apt::ppa { 'ppa:mercurial-ppa/releases': }

# how to use with create_resources?

$ppas = hiera_hash('site_repos_ppa', {})

create_resources('@apt::ppa', $ppas)

gistfile3.txt

# This results in:
Error: Could not retrieve catalog from remote server: 
  Error 400 on SERVER: can't convert nil into Hash at 
  /etc/puppet/environments/production/modules/repos/manifests/virtual.pp:9 
  on node kvmhost01.example.com