Post-fix after ba9f5f4af1ed329128ae04c22952902604ba081d  (adding _server_host attribute).
Post-fix after ba9f5f4af1ed329128ae04c22952902604ba081d  (adding _server_host
attribute).

mysql->host is not always set (apparently not when reconnecting),
This made MariaDB C/C produce invalid handshake reply packet
with incorrecly encoded connection attributes. there cannot be connection
attributes with empty values in MySQL protocol.

Fix connector to only send _server_host is host is valid non-empty string.
Also prevent adding connection attributes with empty values.

ottok/mariadb-connector-c,MariaDB/mariadb-connector-c,ottok/mariadb-connector-c,MariaDB/mariadb-connector-c,ottok/mariadb-connector-c,MariaDB/mariadb-connector-c