opkg: ignore empty fields in package lists

This is to work around issues with package list generators creating
empty fields in some environments.

Based on a patch by Paul Selkrik <pselkrik@isc.org>
Signed-off-by: Steven Barth <steven@midlink.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38390 3c298f89-4303-0410-b956-a3cf2f4a3e73