luci-base: fix some minor luci.model.uci issues
luci-base: fix some minor luci.model.uci issues

 - Properly serialize option delete changelogs
 - Do not perform a section create if a nil value is passed to set()

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

chris5560/openwrt-luci,tobiaswaldvogel/luci,kuoruan/lede-luci,openwrt/luci,wongsyrone/luci-1,chris5560/openwrt-luci,openwrt/luci,chris5560/openwrt-luci,kuoruan/luci,nmav/luci,hnyman/luci,kuoruan/lede-luci,kuoruan/luci,hnyman/luci,hnyman/luci,openwrt/luci,nmav/luci,hnyman/luci,nmav/luci,lbthomsen/openwrt-luci,chris5560/openwrt-luci,lbthomsen/openwrt-luci,nmav/luci,hnyman/luci,wongsyrone/luci-1,kuoruan/lede-luci,openwrt-es/openwrt-luci,lbthomsen/openwrt-luci,chris5560/openwrt-luci,openwrt-es/openwrt-luci,kuoruan/luci,openwrt/luci,openwrt-es/openwrt-luci,tobiaswaldvogel/luci,nmav/luci,hnyman/luci,rogerpueyo/luci,rogerpueyo/luci,kuoruan/luci,chris5560/openwrt-luci,tobiaswaldvogel/luci,openwrt-es/openwrt-luci,wongsyrone/luci-1,tobiaswaldvogel/luci,rogerpueyo/luci,chris5560/openwrt-luci,openwrt/luci,wongsyrone/luci-1,tobiaswaldvogel/luci,artynet/luci,artynet/luci,chris5560/openwrt-luci,rogerpueyo/luci,openwrt-es/openwrt-luci,nmav/luci,artynet/luci,openwrt/luci,kuoruan/luci,kuoruan/luci,artynet/luci,rogerpueyo/luci,Noltari/luci,kuoruan/luci,Noltari/luci,openwrt-es/openwrt-luci,wongsyrone/luci-1,Noltari/luci,openwrt/luci,artynet/luci,wongsyrone/luci-1,artynet/luci,Noltari/luci,openwrt-es/openwrt-luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,artynet/luci,openwrt-es/openwrt-luci,kuoruan/lede-luci,hnyman/luci,Noltari/luci,Noltari/luci,lbthomsen/openwrt-luci,openwrt/luci,rogerpueyo/luci,Noltari/luci,tobiaswaldvogel/luci,hnyman/luci,lbthomsen/openwrt-luci,nmav/luci,kuoruan/lede-luci,kuoruan/lede-luci,lbthomsen/openwrt-luci,rogerpueyo/luci,kuoruan/luci,artynet/luci,kuoruan/lede-luci,nmav/luci,kuoruan/lede-luci,lbthomsen/openwrt-luci,Noltari/luci,tobiaswaldvogel/luci,wongsyrone/luci-1,rogerpueyo/luci,wongsyrone/luci-1,Noltari/luci,nmav/luci,artynet/luci