PDO MySQL: Normalize handling of empty stored procedure result set

PDO MySQL: Normalize handling of empty stored procedure result set

MySQL always returns a trailing empty result set for stored
procedure calls, which is used to convey status information.
The PDO MySQL implementation is presently confused about what to
do with it: If mysqlnd is used and native prepared statements are
used, this result set is skipped. In all other cases it is not
skipped. We also have quite a few XFAILed tests relating to this.

This patch normalizes (for PHP-8.0 only) the behavior towards
always retaining the empty result set. This is simply how MySQL
stored procedures work (some expletives omitted here) and we can't
distinguish this "useless" result set from an empty result of a
multi query. Multi queries are not a concern for native prepared
statements, as PDO does not allow them in that case, but they are
a concern for emulated prepared statements.

Closes GH-6497.