skip render empty lines with blank

Users may want to know whether the corresponding line has spaces, but
the current implementation unconditionally renders blanks based on
indentation.

Signed-off-by: yuguorui <yuguorui@pku.edu.cn>

Author: yuguorui

lua/hlchunk/mods/blank.lua

@@ -46,7 +46,7 @@ function blank_mod:render_line(index, indent)
         local char = line_val:sub(shiftwidth * i - 1, shiftwidth * i - 1)
         if char == "\t" then
             text = text .. ("t") .. (" "):rep(shiftwidth - 1)
-        else
+        elseif char ~= "" then
             text = text .. "." .. (" "):rep(shiftwidth - 1)
         end
     end