watch监听选择器 点击清空按钮时会报错

出现这种情况的原因是: 当点击清除按钮的时候，value会被设置为null。

<el-date-picker
   v-model="value2"
   type="daterange"
   align="right"
   unlink-panels
   range-separator="至"
   start-placeholder="开始日期"
   end-placeholder="结束日期"
   :picker-options="pickerOptions"
 >
 </el-date-picker>

// data数据
  return {
    // 日期
    pickerOptions: {
      shortcuts: [
        {
          text: "最近一周",
          onClick(picker) {
            const end = new Date();
            const start = new Date();
            start.setTime(start.getTime() - 3600 * 1000 * 24 * 7);
            picker.$emit("pick", [start, end]);
          },
        },
        {
          text: "最近一个月",
          onClick(picker) {
            const end = new Date();
            const start = new Date();
            start.setTime(start.getTime() - 3600 * 1000 * 24 * 30);
            picker.$emit("pick", [start, end]);
          },
        },
        {
          text: "最近三个月",
          onClick(picker) {
            const end = new Date();
            const start = new Date();
            start.setTime(start.getTime() - 3600 * 1000 * 24 * 90);
            picker.$emit("pick", [start, end]);
          },
        },
      ],
    },
    value2: [],
  };

// 事件
watch: {
    value2(newData) {
      if (!newData) {
      	// 清空的时候 给它设置为 []
        newData = [];
        this.beginDate = ""; //开始日期
        this.endDate = ""; //结束日期
        this.listSpecificSupplyVO();
      } else {
      	// 将中国标准时间转换
        var d = new Date(newData[0]);
        var n = d.getFullYear();
        var y;
        var r;
        var y1;
        var r1;
        if (d.getMonth() + 1 < 10) {
          y = "0" + (d.getMonth() + 1);
        } else {
          y = d.getMonth() + 1;
        }

        if (d.getDate() < 10) {
          r = "0" + d.getDate();
        } else {
          r = d.getDate();
        }
        //开始时间
        var youWant = n + "-" + y + "-" + r;
       

        var d2 = new Date(newData[1]);
        var n1 = d2.getFullYear();
        if (d2.getMonth() + 1 < 10) {
          y1 = "0" + (d2.getMonth() + 1);
        } else {
          y1 = d2.getMonth() + 1;
        }

        if (d2.getDate() < 10) {
          r1 = "0" + d2.getDate();
        } else {
          r1 = d2.getDate();
        }
		//结束时间
        var youWant2 = n1 + "-" + y1 + "-" + r1;
      }
    },
  },