prometheus-node-exporter-lua: tonumber of empty string causing error and dropped metrics

[shikasta-net]

Maintainer: @champtar
Environment: RTL8382, D-Link DGS-1210-16, Openwrt 23.05.4; and RTL8380, D-Link DGS-1210-10MP F, Openwrt 23.05.5

Description:
Log noise and missing metrics caused at:

packages/utils/prometheus-node-exporter-lua/files/usr/lib/lua/prometheus-collectors/netclass.lua

Lines 36 to 38 in bbc02fa

	if speed ~= nil and tonumber(speed) >= 0 then
	metric_node_network.speed_bytes({device = device}, tonumber(speed)*1000*1000/8)
	end

if speed ~= nil and tonumber(speed) >= 0 then causes

/usr/lib/lua/prometheus-collectors/netclass.lua:36: attempt to compare number with nil

when speed is an empty string, because tonumber('') = nil and nil >= 0 is invalid. Is only an issue on these d-link switches, not linksys wrt3200 router or BT homehub5 accesspoint.

[shikasta-net]

I tried the obvious thing of adding and speed ~= '' to the conditional but that fails with

/usr/bin/prometheus-node-exporter-lua:42: bad argument #4 to 'format' (string expected, got nil)