utils: bufilter: Check for nil

it will be empty if tabufline is disabled

lua/core/utils.lua

@@ -175,7 +175,11 @@ M.packer_sync = function(...)
 end
 
 M.bufilter = function()
-  local bufs = vim.t.bufs
+  local bufs = vim.t.bufs or nil
+
+  if not bufs then
+    return {}
+  end
 
   for i = #bufs, 1, -1 do
     if not vim.api.nvim_buf_is_valid(bufs[i]) then