compatibility with future parser

without this, when gr_memcache_hosts is not used:

Error: Failed to parse template graphite/opt/graphite/webapp/graphite/local_settings.py.erb:
  Filepath: /usr/src/puppet/modules/graphite/templates/opt/graphite/webapp/graphite/local_settings.py.erb
  Line: 55
  Detail: undefined method `each' for nil:NilClass