Revert type hint to match pre-security PR state

Revert _getChartArray type hint from `$chart = null` back to `?WP_Post $chart = null` to match the state BEFORE the broken security PR was merged to development.

The security PR in development has a bug where nonce creation and verification don't match. Our code maintains the working pre-security-PR state.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vytisbulkevicius

classes/Visualizer/Module/Chart.php

@@ -379,7 +379,7 @@ public function getCharts() {
 	 *
 	 * @return array The array of chart data.
 	 */
-	private function _getChartArray( $chart = null ) {
+	private function _getChartArray( ?WP_Post $chart = null ) {
 		if ( is_null( $chart ) ) {
 			$chart = $this->_chart;
 		}