fix(forms): Update types for TypeScript nullability support

This reverts commit 2e47a0d19f.

packages/forms/src/directives/number_value_accessor.ts

@@ -47,7 +47,7 @@ export class NumberValueAccessor implements ControlValueAccessor {
     this._renderer.setElementProperty(this._elementRef.nativeElement, 'value', normalizedValue);
   }
 
-  registerOnChange(fn: (_: number) => void): void {
+  registerOnChange(fn: (_: number|null) => void): void {
     this.onChange = (value) => { fn(value == '' ? null : parseFloat(value)); };
   }
   registerOnTouched(fn: () => void): void { this.onTouched = fn; }