undef does not equal '' with future parser

With future parser an undef string does not == an empty string. Since the case statement also does not include the special word `undef` set $inline_template = '' to work correctly with the case statement.

see: https://tickets.puppetlabs.com/browse/PUP-1807
Merge pull request #10 from derektamsen/patch-1

undef does not equal '' with future parser