verify that document requests work without didOpen/didClose events coming in

[martinlippert]

According to the discussion in microsoft/language-server-protocol#1912, I stumbled upon the fact that document related requests (e.g. for code lenses or other stuff) can happen without a prior didOpen event (and corresponding didClose).

We need to double check and verify that the Spring Boot language server behaves correctly according to this, so that it can serve those document-related requests also in situations where there is no document open and therefore not in the internal document service store.