fetch: convert a NULL property query to ""

fetch: convert a NULL property query to ""

Previously, a NULL property query was never cached and this lead to a
performance degregation.  Now, such a query is converted to an empty string
and cached.

Fixes #17752
Fixes https://github.openssl.org/openssl/openssl/issues/26

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17769)