fix: don't freak out when not passed a `finders` config
fix: don't freak out when not passed a `finders` config

As reported here:

    https://github.com/wincent/command-t/issues/393#issuecomment-1229168699

if `options.finders` is not set to (at least an empty) table, we freak
out with:

    lua/wincent/commandt/init.lua:343: bad argument #1 to 'pairs' (table expected, got nil)

wincent/command-t,wincent/command-t,wincent/command-t