refactor(server): Prefer `null` over `undefined` and explicit `void` (#1493)
refactor(server): Prefer `null` over `undefined` and explicit `void` (#1493)

Update return types of functions to:

* Explicitly have `void` as the return type
* Prefer `null` over `undefined`

The LSP spec always uses `null` as the return types:
https://microsoft.github.io/language-server-protocol/specifications/specification-current/

In fact, if the server is configured to return `undefined`, it will
still be `null` after coming through the LSP connection. To avoid any
confusion, we should just always use `null`.
angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service