[filename] avoid new file status for unnamed (#795)

lua/lualine/components/filename.lua

@@ -29,7 +29,8 @@ end
 
 local function is_new_file()
   local filename = vim.fn.expand('%')
-  return vim.bo.buftype == '' and vim.fn.filereadable(filename) == 0
+  return filename ~= '' and vim.bo.buftype == '' and
+    vim.fn.filereadable(filename) == 0
 end
 
 ---shortens path by turning apple/orange -> a/orange