fix: nil check for setup config

[IfanTsai]

Check nil for user setup config. If user do not pass config for setup likes require("rsync").setup(), the error will occurs

E5108: Error executing lua .../.local/share/nvim/lazy/rsync.nvim/lua/rsync/config.lua:26: bad argument #1 to 'pairs' (table expected, got nil)
stack traceback:
        [C]: in function 'pairs'
        .../.local/share/nvim/lazy/rsync.nvim/lua/rsync/config.lua:26: in function 'apply_config'
        .../.local/share/nvim/lazy/rsync.nvim/lua/rsync/init.lua:43: in function 'setup'
        .../.config/nvim/lua/usr/init.lua:67: in main chunk
        [C]: in function 'require'
        [string ":lua"]:1: in main chunk

[OscarCreator]

Looks good!