message: fix: Null values on Treemap right-click
diff: import { DataRecordValue, QueryObjectFilterClause } from '@superset-ui/core';
  import React, { useCallback } from 'react';
  import Echart from '../components/Echart';
  +import { NULL_STRING } from '../constants';
  import { EventHandlers } from '../types';
  import { extractTreePathInfo } from './constants';
  import { TreemapTransformedProps } from './types';
  @@ -97,7 +98,7 @@ export default function EchartsTreemap({
  filters.push({
  col: groupby[i],
  op: '==',
  - val: path,
  + val: path === 'null' ? NULL_STRING : path,
  formattedVal: path,
  }),
  );
  