fix(frontend): チャートのlegendがクリックに反応しない問題を修正

これにより発生 https://github.com/misskey-dev/misskey/pull/12926

packages/frontend/src/components/MkChartLegend.vue

@@ -30,9 +30,9 @@ function onClick(item: LegendItem) {
 	if (chart.value == null) return;
 	if (type.value === 'pie' || type.value === 'doughnut') {
 		// Pie and doughnut charts only have a single dataset and visibility is per item
-		if (item.index) chart.value.toggleDataVisibility(item.index);
+		if (item.index != null) chart.value.toggleDataVisibility(item.index);
 	} else {
-		if (item.datasetIndex) chart.value.setDatasetVisibility(item.datasetIndex, !chart.value.isDatasetVisible(item.datasetIndex));
+		if (item.datasetIndex != null) chart.value.setDatasetVisibility(item.datasetIndex, !chart.value.isDatasetVisible(item.datasetIndex));
 	}
 	chart.value.update();
 }