EVP_*Update: ensure that input NULL with length 0 isn't passed

Even with custome ciphers, the combination in == NULL && inl == 0
should not be passed down to the backend cipher function.  The reason
is that these are the values passed by EVP_*Final, and some of the
backend cipher functions do check for these to see if a "final" call
is made.

Fixes #8675

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8676)