prometheus-node-exporter-lua: handle empty lines in all_sta output correctly

An empty line has a name and value that is nil and setting a table
index to nil breaks metrics for every vif after the first one.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>

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

@@ -129,7 +129,9 @@ local function scrape()
         current_station_values = {}
       else
         local name, value = string.match(line, "(.+)=(.+)")
-        current_station_values[name] = value
+        if name ~= nil then
+          current_station_values[name] = value
+        end
       end
     end
     labels.station = current_station