luci-mod-status: gracefully handle missing port stats

Ref: https://github.com/openwrt/luci/commit/cba58fcafbe3a3f4f599354f7c123b546e045887#commitcomment-123043170
Fixes: 4e466248

 ("luci-mod-status: introduce ethernet port status view")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>