message: Completion requests should return an empty array instead of null
diff: @@ -27,6 +27,7 @@ import {
  ServerCapabilities,
  } from 'vscode-jsonrpc';
  import {
  + CompletionItem,
  CompletionRequest,
  CompletionList,
  DefinitionRequest,
  @@ -279,9 +280,9 @@ export class MessageProcessor {
  async handleCompletionRequest(
  params: CompletionRequest.type,
  token: CancellationToken,
  - ): Promise<CompletionList> {
  + ): Promise<CompletionList | Array<CompletionItem>> {
  if (!this._isInitialized) {
  - return null;
  + return [];
  }
  // `textDocument/comletion` event takes advantage of the fact that
  // `textDocument/didChange` event always fires before, which would have
  @@ -315,7 +316,7 @@ export class MessageProcessor {
  // If there is no GraphQL query in this file, return an empty result.
  if (!found) {
  - return null;
  + return [];
  }
  const {query, range} = found;
  