Avoid empty strings when --with-packager(-version) is not specified

Avoid empty strings when --with-packager(-version) is not specified

Make with_packager and with_packager_version default to "no". This way
--without-packager-version (as shorthand for --with-packager(-version)=no)
works correctly too.

Prior to this patch libvirt outputs a line like this when
--with-packager(-version) was not specified

# ./daemon/libvirtd
14:11:15.018: 31796: info : libvirt version: 0.8.8, package:  ()

Now the unspecified parts are correctly omitted.

Reported by Osier Yang.