correct typings of simple states

src/Umbraco.Web.UI.Client/libs/observable-api/string-state.ts

@@ -3,9 +3,11 @@ import { BasicState } from "./basic-state";
 /**
  * @export
  * @class StringState
- * @extends {BehaviorSubject<T>}
+ * @extends {BasicState<T>}
  * @description - A RxJS BehaviorSubject this Subject ensures the data is unique, not updating any Observes unless there is an actual change of the value.
  */
-export class StringState<T extends string | undefined | null> extends BasicState<T> {
-
+export class StringState<T> extends BasicState<T | string> {
+	constructor(initialData: T | string) {
+		super(initialData);
+	}
 }