Add null | undefined to parse results. Fixes #937

[danielnixon]

Fixes #937.

This PR might be too much of a swing in the opposite direction. The reason I've included null and undefined in all props is because it's difficult to trace exactly which props are always assigned a value versus which might be null versus which might be null or undefined.

I've got a branch locally where I'm slowly annotating everything with types, but that is slow going. In the meantime, erring this way is arguably more appropriate.

[danielnixon]

An alternative option here (slightly more invasive) could be to coalesce everything in the object returned from parse with either null or undefined, then only include one or the other in the type definition. Something like:

return {
      title: this._articleTitle ?? undefined,
      byline: metadata.byline || this._articleByline ?? undefined,
      dir: this._articleDir ?? undefined,
      lang: this._articleLang ?? undefined,
      content: this._serializer(articleContent) ?? undefined,
      textContent: textContent  ?? undefined,
      length: textContent.length  ?? undefined,
      excerpt: metadata.excerpt  ?? undefined,
      siteName: metadata.siteName || this._articleSiteName  ?? undefined,
      publishedTime: metadata.publishedTime  ?? undefined,
    };

(or null instead of undefined if that is the preference).

[cmkm]

Thank you! If this doesn't work for yourself or other Typescript users in the future we can revisit this with your alternate solution.