Describe the bug
Heatmap chart with colorScale={ScaleType.Linear} ends up with duplicate 0 in legend items.

To Reproduce
Provide a dataset with values range from 0 to N.

See https://codesandbox.io/s/damp-https-2rspz?file=/src/App.tsx

Expected behaviour
Legend doesn't contain duplicate items.

Screenshots

Kibana Cross Issues
elastic/kibana#97978