lexer would hang on an empty file

ldoc/lexer.lua

@@ -156,11 +156,12 @@ function lexer.scan (s,matches,filter,options)
         local line = 1
         if file then
             s = file:read()
-            if not s then return nil end -- empty file
+            if not s or s == '' then return nil end -- empty file
             s = s ..'\n'
          end
         local sz = #s
         local idx = 1
+        if sz == 0 then return nil end -- empty file
         --print('sz',sz)
         while true do
             for _,m in ipairs(matches) do

ldoc/prettify.lua

@@ -39,6 +39,7 @@ function prettify.lua (file)
 
    local tok = lexer.lua(code,{},{})
    local t,val = tok()
+   if not t then return nil,"empty file" end
    while t do
       val = escape(val)
       if spans[t] then