fix: ensure nil checks before Neovim API calls

[argizuno]

fix an issue where closing a window occasionally triggers an error due to an invalid window reference:

Error detected while processing BufLeave Autocommands for "<buffer=2>":
Error executing lua callback: ...y/render-markdown.nvim/lua/render-markdown/core/util.lua:38: Invalid 'window': Expected Lua number
stack traceback:
        [C]: in function 'nvim_win_is_valid'
        ...y/render-markdown.nvim/lua/render-markdown/core/util.lua:38: in function 'valid'
        ...azy/render-markdown.nvim/lua/render-markdown/core/ui.lua:80: in function 'update'
        ...azy/render-markdown.nvim/lua/render-markdown/manager.lua:77: in function <...azy/render-markdown.nvim/lua/render-markdown/manager.lua:70>

[MeanderingProgrammer]

Thanks for the PR!

[MeanderingProgrammer]

I updated the logic to run the nil check prior to calling update: eb8fdac

Just so the type annotations are still meaningful, if the issue crops up again please LMK!

[argizuno]

It seems to be working! Thank you, and keep up the great work!