Fix OSSL_PARAM creation in OSSL_STORE_open_ex

Fix OSSL_PARAM creation in OSSL_STORE_open_ex

The params[0].data is set to a non-NULL value, but params[0].data_size
is always zero. This confuses get_string_internal, which creates 1 byte
string with uninitialized content.

When OSSL_PARAM_construct_utf8_string is used, the data_size is set
correctly.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13699)