在ng-bootstrap表中，当表列包含null或空值(角度为8)时，排序不一致

import { Component } from '@angular/core';
import { SortType } from 'ngx-bootstrap';

function customSortFunction(value1: any, value2: any, sortType: SortType): number {
  if (value1 === null || value1 === '') {
    return sortType === SortType.DESC ? 1 : -1;
  } else if (value2 === null || value2 === '') {
    return sortType === SortType.DESC ? -1 : 1;
  } else {
    // 默认使用默认排序方式
    return value1.localeCompare(value2);
  }
}

<ngb-table #table [columns]="columns" [data]="data">
  <ng-template ngFor [ngForOf]="columns" let-column let-i="index">
    <ng-template [ngIf]="column.sortable">
      <th
        [ngbSortable]="column.property"
        (sort)="table.sort(column.property, table.getSortDirection(column.property, i), customSortFunction)"
        [class.active]="table.isSorted(column.property)"
        [class.asc]="table.isSortedAscending(column.property)"
        [class.desc]="table.isSortedDescending(column.property)">
        {{ column.header }}
      </th>
    </ng-template>
    <ng-template [ngIf]="!column.sortable">
      <th>{{ column.header }}</th>
    </ng-template>
  </ng-template>

  <tbody>
    <tr *ngFor="let row of table.rows">
      <td *ngFor="let cell of row.cells">{{ cell.value }}</td>
    </tr>
  </tbody>
</ngb-table>