(FACT-379) Don't return VLANs when only header is present

On CentOS 5 the vlans file is always present regardless of if any vlans
is defined, which behaves like there are VLANs defined but doesn't match
any regexps. This commit explicitly ensures that we have a non-empty
list of VLANs before returning anything.
Merge branch 'bugfix/facter-2/fact-379-centos-5-vlans' into stable