fix: handle cancellation of vim.ui.select by user

[smjonas]

Previously, closing the vim.ui.select window, e.g. with <esc> would cause this error:

Error executing vim.schedule lua callback: vim/shared.lua:0: after the second argument: expected table, got nil                                                                                                                                                 
stack traceback:                                                                                                                                                                                                                                                
        [C]: in function 'error'                                                                                                                                                                                                                                
        vim/shared.lua: in function 'validate'                                                                                                                                                                                                                  
        vim/shared.lua: in function 'tbl_deep_extend'                                                                                                                                                                                                           
        ...e/jonas/.local/share/nvim/lazy/gen.nvim/lua/gen/init.lua:192: in function 'cb'                                                                                                                                                                       
        ...e/jonas/.local/share/nvim/lazy/gen.nvim/lua/gen/init.lua:172: in function 'on_choice'                                                                                                                                                                
        ...are/nvim/lazy/dressing.nvim/lua/dressing/select/init.lua:78: in function ''                                                                                                                                                                          
        vim/_editor.lua: in function <vim/_editor.lua:0>     

[David-Kunz]

Great, thank you @smjonas !