Avoid looping forever when hook is called with unchanged_cert

[bscattergood]

Ubuntu 18.04 has dehydrated 0.6.1 in the default repositories.

When running "dehydrated -c" and there are no certificates to update,
that calls the hook with "unchanged_cert" and 5 additional parameters.

The hook script then hangs (as it will when run when the number of
arguments isn't a multiple of 3), since "$1" is not empty, but "shift
3" fails and leaves the arguments alone.

Avoid this by checking if "$3" is empty instead.