ossl_method_store_cache_get(): ensure non-NULL property query
ossl_method_store_cache_get(): ensure non-NULL property query

The comparator further down the call stack doesn't tolerate NULL, so
if we got that as input, use the empty string.

Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8781)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl