Fix #20525 - Undefined global variable $maxsize - #20587

Description

When an export request is submitted without the maxsize parameter ("Skip tables larger than"), Export::exportDatabase() reads $GLOBALS['maxsize'], which was never set, because ExportController only copies POST parameters that are present into the globals. On PHP 8.1+ this triggers "Undefined global variable $maxsize" (reported at https://reports.phpmyadmin.net/incidents/view/2969944).

This defaults the value to an empty string, which the existing check already treats as "no size limit". A regression test is added to ExportTest.

Fixes #20525

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Replaces #20526, which got closed when my fork was accidentally detached.