[Bug] 使用customMergeCell自定义合并单元格时，合并后的单元格显示值text属性设置为空字符串时，合并单元格功能不生效

[KzqKzq]

Version

1.4.2-alpha.3

Link to Minimal Reproduction

https://codesandbox.io/p/sandbox/pagination-forked-dsxzpp?file=%2Fsrc%2Findex.js%3A11%2C36

Steps to Reproduce

使用customMergeCell自定义合并单元格，customMergeCell的函数返回

customMergeCell = (col, row, table) => {
  if (row >= 4 && col <= 5 && col === 0) {
    return {
      text: '',
      range: {
        start: {
          col: 0,
          row: 4,
        },
        end: {
          col: 0,
          row: 6,
        },
      },
    };
  }
}

### Current Behavior

![20240703142353_rec_](https://github.com/VisActor/VTable/assets/61263079/df371bf6-5c73-4ca1-b0e3-acefc1ddf6c5)


### Expected Behavior

text 填空字符串时可以正常合并单元格

### Environment

```markdown
- OS:window 10
- Browser: Chrome 126.0.6478.127
- Framework:React 16 / 原生JS

Any additional comments?

No response