fix(项目报告): 为饼状图柱状图设置白色背景色避免导出了背景为灰色的图片

--bug=1018509 --user=宋天阳 【报表统计】项目报表-测试用例统计导出图标优化图标背景
https://www.tapd.cn/55049933/s/1269760

report-stat/frontend/src/business/projectstatistics/casecount/TestCaseCountContainer.vue

@@ -121,6 +121,7 @@ export default {
       this.resetOptions();
       if (barData) {
         this.formatData(barData);
+        this.loadOption.backgroundColor = "#FFFFFF";
         this.loadOption.legend = barData.legend;
         this.loadOption.xAxis = barData.xaxis;
         this.loadOption.xaxis = barData.xaxis;
@@ -136,6 +137,7 @@ export default {
       }
       if (pieData) {
         this.formatData(pieData);
+        this.pieOption.backgroundColor = "#FFFFFF";
         this.pieOption.legend = pieData.legend;
         this.pieOption.series = pieData.series;
         this.pieOption.title = pieData.title;