Fix nil value emergence

Author: archy-co

cpu_rings.lua

@@ -161,12 +161,14 @@ end
 function conky_clock_rings()
     local function setup_rings(cr,pt)
         local str=''
-        local value=0
+        local value=1
 
         str=string.format('${%s %s}',pt['name'],pt['arg'])
         str=conky_parse(str)
 
-        value=tonumber(str)
+        if(str ~= '') then
+            value=tonumber(str)
+        end
         pct=value/pt['max']
 
         draw_ring(cr,pct,pt)