getApplicableRefactors: Don't return undefined response

[ghost]

In session.ts:

[CommandNames.GetApplicableRefactors]: (request: protocol.GetApplicableRefactorsRequest) => {
    return this.requiredResponse(this.getApplicableRefactors(request.arguments));
},

If we return undefined for something with a required response, this will cause an error later on. (The error message is simply "No content available.".)

There is probably a similar error for getEditsForRefactor.

[ghost]

Actually, it looks like several other services (completions, references, definition) can return an undefined value but will then fail with "No content available." This is in cases where e.g. no definition is available, but should it really be considered an "error" from a services standpoint? It's making it hard to detect actual errors in large tsserver logs. Is there some reason for services to return undefined instead of an empty array (which produces no error)? CC @mjbvz @mhegazy