more future parser improvements

as per subject.

without them:

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: 199
  Detail: undefined method `join' for nil:NilClass

(plus other scenarios with certain variables)