luci-base: fix dispacher fail
luci-base: fix dispacher fail

http.getenv("SCRIPT_NAME") fail if it's not provided. This can happen in the login screen when we don't have any script to load.

Signed-off-by: Ansuel Smith <8c645932af3224b0115bbb0f5493ff0cc8fff0b2@gmail.com>

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