Avoid null as return type

[javagl]

Addresses #162

Builds upon #167 - will be processed further after this one is merged.

There have been a few places that used null as a return type. While there are possible justifications for that, it was hard to ensure consistency in that regard (and it seems to be pretty uncommon in other codebases like CesiumJS). This PR changes the use of these null values to undefined.

The affected places:

• ResourceResolver.resolve... functions returned null when the data could not be resolved, but return undefined now
• ContentData.getData (which usually contained the data that was obtained with a ResourceResolver) could be null, and can now be undefined accordingly
• The TreeCoordinates interface returned null as the parent of root nodes. This also returns undefined now

(The other changes are the respective implementations and places where these values had been used)

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Note: There is still a large part of the codebase that does use null - namely, the implementations of the glTF extensions based on glTF-Transform. This is because glTF-Transform internally does use null, pretty consistently. So it makes sense to use null in these implementations as well.