Bugfix check if windows exist before resizing

[sandergs92]

📃 Summary

With the nvim command:
:q

An error happens:

Error detected while processing WinResized Autocommands for "*": Error executing lua callback: .../share/nvim/lazy/presenting.nvim/lua/presenting/init.lua:216: Invalid window id: 1003 stack traceback: [C]: in function 'nvim_win_set_config' .../share/nvim/lazy/presenting.nvim/lua/presenting/init.lua:216: in function 'resize' .../share/nvim/lazy/presenting.nvim/lua/presenting/init.lua:31: in function <.../share/nvim/lazy /presenting.nvim/lua/presenting/init.lua:31>

Small fix that checks windows existence first before resizing. Even though, most user behavior should be to quit by doing:
q