Fix sudoers.erb to stop writing an invalid "Defaults:" line if no defaults given

Commit b17f148def17b6f12a74bb59d0b6b6fa0e721142 introduced a bug where leaving `defaults` as the default (an empty array) causes an invalid line to be written to the sudoers file. This happens because an empty array is considered to be `true` in Ruby, so the `if` check in the template will always be satisfied unless you explicitly pass `defaults => undef`. At least, that's how it works for me in Puppet v3.3.1. 

Sudo really doesn't like syntax errors, and will stop working with an error like the following:

    sudo: >>> /etc/sudoers.d/admins: syntax error near line 7 <<< 
    sudo: parse error in /etc/sudoers.d/admins near line 7
    sudo: no valid sudoers sources found, quitting
    sudo: unable to initialize policy plugin

If you don't have root access to the server, you'll be locked out of administering it at this point.