fix(tabufline): nil check when tabufline is disabled

[CoreyCole]

when pressing shift-tab and tab when tabufline is disabled we get errors because there is no vim.t

tabufline = {
  enabled = false,
},

handle vim.t.bufs nil:

E5108: Lua: .../.local/share/nvim/lazy/ui/lua/nvchad/tabufline/init.lua:11: bad argument #1 to 'ipairs' (table expected, got nil)

[siduck]

but those who don't use the tabufline then they should disable the TAB mapping itself

[CoreyCole]

is there a way we can prevent creating the mapping automatically when tabufline is disabled? seems like the mapping is created in a different repo not ui?

[siduck]

done