message: Fix error thrown by `:set` with no args
  This still throws an error (since it's not implemented yet), but at least it's an expected error.
  Fixes
diff: import * as node from '../commands/setoptions';
  import { Scanner } from '../scanner';
  -export function parseOption(args: string): node.IOptionArgs {
  - let scanner = new Scanner(args);
  +export function parseOption(args?: string): node.IOptionArgs {
  + let scanner = new Scanner(args ?? '');
  scanner.skipWhiteSpace();
  if (scanner.isAtEof) {
  