Add nullability annotations

[NickAcPT]

Several methods and are missing nullability annotations (@NonNull or @Nullable).

This results in a poor experience in languages like Kotlin because method arguments and return types do not contain the correct nullable or non-null types.

[jonahgraham]

Hi @NickAcPT - I would be happy to review/merge a pull request that added such annotations. It may be best to do a single file first to make sure we are all on the same page. For example, the @NonNull annotation in the code today is org.eclipse.lsp4j.jsonrpc.validation.NonNull which may not be what you expect?