MDEV-32627 Distinguish between absence of a keyword and empty value for the keyword

Distinguish them in two place:

when constructing connection key in create_conn_key and
spider_create_conn for both ordinary queries and spider_direct_sql

For spider_create_conn and spider_udf_direct_sql_create_conn, the
created conn gets assigned a field from the source object if and only
if source->field is non-null.

For spider_create_conn_keys and spider_udf_direct_sql_create_conn_key,
we update the encoding so that absence of keyword and keyword with an
empty value result in different keys. More specifically, if the i-th
keyword has a value, the corresponding part in the conn key begins
with the char \i, followed by the possibly empty value and ends with a
\0. If the i-th keyword is not specified, then it does not get a
mention in the conn key.

As part of this change, we also update table param / option parsing to
recognise a singleton empty string when creating an string list,
instead of writing it off as NULL.

storage/spider/spd_table.h

@@ -169,6 +169,8 @@ void spider_print_keys(
 );
 #endif
 
+void spider_create_conn_key_add_one(int* counter, char** target, char* src);
+
 int spider_create_conn_keys(
   SPIDER_SHARE *share
 );