Avoid null as return type

[javagl]

The ResourceResolver interface is intended as an abstraction for file systems, databases, or web requests. It offers the functionality to just throw in a (relative) URI/path as a string, and receive a Buffer with the data.

For some reason, I once decided to use null as the return type indicating that "this thing is not threre". This is the only place where null is used in the codebase. But I don't see a strong reason do do this. So I'd change this from null to undefined.

This may not happen in the next "patch" release (0.4.x). But it may happen in the next "semi-major" release (i.e. 0.5.0), which may contain other "breaking changes".