fix(plugin-chart-echarts): show zero value in tooltip (#21296)

Co-authored-by: Ville Brofeldt <ville.brofeldt@apple.com>

superset-frontend/plugins/plugin-chart-echarts/src/utils/forecast.ts

@@ -16,6 +16,7 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+import { isNumber } from 'lodash';
 import { DataRecord, DTTM_ALIAS, NumberFormatter } from '@superset-ui/core';
 import { OptionName } from 'echarts/types/src/util/types';
 import { TooltipMarker } from 'echarts/types/src/util/format';
@@ -60,7 +61,7 @@ export const extractForecastValuesFromTooltipParams = (
     const { marker, seriesId, value } = param;
     const context = extractForecastSeriesContext(seriesId);
     const numericValue = isHorizontal ? value[0] : value[1];
-    if (numericValue) {
+    if (isNumber(numericValue)) {
       if (!(context.name in values))
         values[context.name] = {
           marker: marker || '',
@@ -94,7 +95,7 @@ export const formatForecastTooltipSeries = ({
 }): string => {
   let row = `${marker}${sanitizeHtml(seriesName)}: `;
   let isObservation = false;
-  if (observation) {
+  if (isNumber(observation)) {
     isObservation = true;
     row += `${formatter(observation)}`;
   }