[Feature] Be able to set the cell's backgroundColor when chart type is heatmap

### What problem does this feature solve?

When I use the Heatmap chart, i need to set a backgroundColor when cell's value is null

### What does the proposed API look like?

`
      {
        type: 'heatmap',
        data: chartData,
        itemStyle: {
          backgroundColor: '#A8A8A8',
        },
...
}`