params: do not ignore zero-length strings

params: do not ignore zero-length strings

Prior to this commit, if a string (or octet string) parameter
was present but indicated it was zero-length, we would return success
but with a NULL output value.  This can be problematic in cases where
there is a protocol-level distinction between parameter-absent and
parameter-present-but-zero-length, which is uncommon but can happen.

Since OPENSSL_malloc() returns NULL for zero-length allocation requests,
make a dummy allocation for this case, to give a signal that the string
parameter does exist but has zero length.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11920)