fix: handle error handling of unknown storage descriptions (fixes #11)

[Maximvdw]

null check implementation

[woutslabbinck]

It does indeed make sense that if there are no link headers to Description Resources (.meta resources), the method findService returns null gracefully.

I think for the code to make perfect sense, discoverStorageDescription its type must also be changed to Promise<string | null> to indicate that not every resource has a Description Resource.

Other than that, it looks good to me and can be merged given that change.
After the merge, @jaxoncreed will have to bump/publish the package on npm as he is the only person with that permission.

[Maximvdw]

I will make the requested changes. Ill keep you updated