Strict null checks with datepicker NgbDateStruct

[AlexKhymenko]

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Bug description:

Uncompatible with typescript strict null checks Support. The parse method should return null or NgbDateStruct. Right now return only NgbDateStruct.

Link to minimally-working plunker that reproduces the issue:

You can fork a plunker from one of our demos and use it as a starting point.
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https://github.com/ng-bootstrap/ng-bootstrap#you-think-youve-found-a-bug

Version of Angular, ng-bootstrap, and Bootstrap:

Angular: 4

ng-bootstrap:

Bootstrap:

[AlexKhymenko]

For someone who has this issue but want to use strict null checks the workaround is to add any before null

    if (!value) {
        return <any>null;
    }
    return d.isValid() ? { year: d.year(),
            month: d.month() + 1,
            day: d.date() } : <any>null; 

Not beautiful but works :-)

[pkozlowski-opensource]

Yeh, we should review this project for the proper strict null checks...

[wesleycho]

So I took a stab at trying to implement proper strict null checks...this is not an easy task. This pollutes a lot of areas, and needs a lot of love.