minor fix

src/models/gauge_options.ts

@@ -145,7 +145,7 @@ export class GaugeOptions extends CoreOptions<GaugeConfig> {
     return this._options.outerRadius;
   }
 
-  valueFormatter(value: number): string {
+  valueFormatter(value?: number | null): string {
     if(this._options.valueFormatter) {
       return this._options.valueFormatter(value);
     }