message: return an empty array instead of null for custom directives
diff: @@ -97,14 +97,14 @@ export class GraphQLConfig implements GraphQLConfigInterface {
  return this._getPropertyFromConfig('schemaPath', appName, null);
  }
  - getCustomDirectives(appName: ?string): ?Array<string> {
  + getCustomDirectives(appName: ?string): Array<string> {
  const appConfig = this._getPropertyFromConfig(
  EXTENSIONS_NAME,
  appName,
  null,
  );
  if (appConfig && appConfig[CUSTOM_DIRECTIVES_NAME]) {
  - return appConfig[CUSTOM_DIRECTIVES_NAME];
  + return appConfig[CUSTOM_DIRECTIVES_NAME] || [];
  }
  // As a default fallback, search the top level extensions
  const fallbackExtension = this._getPropertyFromConfig(
  @@ -113,7 +113,7 @@ export class GraphQLConfig implements GraphQLConfigInterface {
  null,
  );
  - return fallbackExtension ? fallbackExtension[CUSTOM_DIRECTIVES_NAME] : null;
  + return fallbackExtension ? fallbackExtension[CUSTOM_DIRECTIVES_NAME] : [];
  }
  isFileInIncludeDirs(fileName: Uri, appName: ?string): boolean {
  