fix: allow for null values in HTML select options bound with ngValue

This corrects the case of <option [ngValue]="null"> binding a string like "{0: null}" to the model instead of an actual null object.

Closes #10349

modules/@angular/forms/src/directives/select_multiple_control_value_accessor.ts

@@ -121,8 +121,8 @@ export class SelectMultipleControlValueAccessor implements ControlValueAccessor
 
   /** @internal */
   _getOptionValue(valueString: string): any {
-    const opt = this._optionMap.get(_extractId(valueString));
-    return opt ? opt._value : valueString;
+    let key: string = _extractId(valueString);
+    return this._optionMap.has(key) ? this._optionMap.get(key)._value : valueString;
   }
 }