Fix failed domainname and searchpath clash

If you have domainname set with no searchpath defined, the domain parameter would not get set in /etc/resolv.conf.

There are a couple of solutions to this problem, one of which it to use the empty() function from stdlib, but given that this class doesn't already use it, I opted for changing default searchpath parameter to undef instead of [] and then check for this in the template. 

It may be that this behaviour only occurs with future parser, but this commit should work with either parser.