Fix Chart.php type hint for PHP compatibility

Remove WP_Post type hint from _getChartArray method to match development branch and prevent PHP errors. A typed parameter cannot have a default value of null without using nullable operator.

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

Author: vytisbulkevicius

classes/Visualizer/Module/Chart.php

@@ -375,11 +375,11 @@ public function getCharts() {
 	 *
 	 * @access private
 	 *
-	 * @param WP_Post $chart The chart object.
+	 * @param WP_Post|null $chart The chart object.
 	 *
 	 * @return array The array of chart data.
 	 */
-	private function _getChartArray( WP_Post $chart = null ) {
+	private function _getChartArray( $chart = null ) {
 		if ( is_null( $chart ) ) {
 			$chart = $this->_chart;
 		}