check for null and undefined instead of boolean

Author: sluger

src/controllers/base.js

@@ -19,7 +19,7 @@ export const horizontalDefaults = {
 
 export function toFixed(value) {
   const decimals = this._chart.config.options.tooltipDecimals; // inject number of decimals from config
-  if (!decimals || typeof decimals !== 'number' || decimals < 0) {
+  if (decimals == null || typeof decimals !== 'number' || decimals < 0) {
     return value;
   }
   return Number.parseFloat(value).toFixed(decimals);