Typeahead checks if falsy but tries to get .length of potentially null value and that throws an error

[matjazonline]

Bug description:

The issue is at 'const count = results.length;' typeahead.ts:360 when 'results' value is null.
The workaround is to pass empty array to items observable, but at the top of this method null is expected: 'if (!results || results.length === 0) {'

Console error:

global-error-handler.ts:37 client error: TypeError: Cannot read property 'length' of null
at SafeSubscriber._next (typeahead.js:238)
at SafeSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub (Subscriber.js:195)
at SafeSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.next (Subscriber.js:133)
at ...

[pkozlowski-opensource]

Here is a reproduction scenario: https://stackblitz.com/edit/angular-metn4u?file=app/typeahead-basic.ts

We do assume that empty results should be denoted by [] but maybe we should protect against null / undefined / not an array...