fix: handle when there's only one tabpages

[phanen]

When there's only one neogit tabpage, type q then do motion cause error:

nvim --headless +Neogit +'sil! tabo | sil! only' --cmd 'lua vim.defer_fn(function() vim.api.nvim_input("qj") end, 1000)'

E5108: Error executing lua: .../neogit/lua/neogit/buffers/status/actions.lua:470: attempt to index field 'buffer' (a nil value)
stack traceback:
               .../neogit/lua/neogit/buffers/status/actions.lua:470: in function 'cb'
               .../neogit/lua/neogit/lib/buffer.lua:745: in function </home/phan/lazy/neogit/lua/neogit/lib/buffer.lua:744>