typeahead: Add '| undefined' to ngbTypeahead typings

[AbakumovAlexandr]

Bug description:

Current declaration in typeahead.d.ts is:

ngbTypeahead: (text: Observable<string>) => Observable<readonly any[]>;

i.e. a function must be provided at all times.

According to this typing, if I'd like to have ngbTypeahead working in some cases and disable it in the others, I have to either provide a noop function or use 2 different inputs in a template switched by *ngIf: one with the [ngbTypeahead] directive and another - without it.

Suppose, I have my own component which wraps an input with the [ngbTypeahead] directive:

<input type="text" [ngbTypeahead]="search"/>

My component must be able to support 2 modes: typeahead-enabled mode and just simple text input mode. Most natural way is to declare an undefune-able @Input like the below:

    @Input()
    public autoCompleteItemsFn: ((text: Observable<string>) => Observable<readonly unknown[]>) | undefined;

(where undefined means 'simple input mode') and bind the [ngbTypeahead] directive to it:

<input type="text" [ngbTypeahead]="autoCompleteItemsFn"/>

Which perfectly worked before even if autoCompleteItemsFn === undefined.

However. I can't do this anymore with strict templates due to incomplete typing.

Could you fix it by changing the typing as follows:

ngbTypeahead: (text: Observable<string>) => Observable<readonly any[]> | undefined;

?

Versions of Angular, ng-bootstrap and Bootstrap:

Angular: 10.2.3

ng-bootstrap: 7.0.0

Bootstrap: 4.5.3