Workaround for mysql2 prepared statements flaky test

Ref: brianmario/mysql2#1383

The proper fix can only be upstream, however we can reduce the likeliness
of hitting the bug by only calling `#affected_rows` when strictly necessary,
meaning when we performed a query that didn't return a result set.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

activerecord/lib/active_record/connection_adapters/mysql2/database_statements.rb

@@ -51,7 +51,12 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
             result = if binds.nil? || binds.empty?
               ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
                 result = raw_connection.query(sql)
-                @affected_rows_before_warnings = raw_connection.affected_rows
+                # Ref: https://github.com/brianmario/mysql2/pull/1383
+                # As of mysql2 0.5.6 `#affected_rows` might raise Mysql2::Error if a prepared statement
+                # from that same connection was GCed while `#query` released the GVL.
+                # By avoiding to call `#affected_rows` when we have a result, we reduce the likeliness
+                # of hitting the bug.
+                @affected_rows_before_warnings = result&.size || raw_connection.affected_rows
                 result
               end
               result