message: Fix bug where empty Swift dictionaries should be '[:]' instead of
  empty string.
diff: @@ -123,8 +123,8 @@ export function classDeclarationForOperation(
  generator.withinBlock(() => {
  generator.printOnNewline(wrap(
  `return [`,
  - join(properties.map(({ name, propertyName }) => `"${name}": ${propertyName}`), ', '),
  - `]`
  + join(properties.map(({ name, propertyName }) => `"${name}": ${propertyName}`), ', ') || ':',
  + `]`,
  ));
  });
  } else {
  @@ -364,7 +364,7 @@ export function dictionaryLiteralForFieldArguments(args) {
  } else if (typeof value === 'object') {
  return wrap('[', join(Object.entries(value).map(([key, value]) => {
  return `"${key}": ${expressionFromValue(value)}`;
  - }), ', '), ']');
  + }), ', ') || ':', ']');
  } else {
  return JSON.stringify(value);
  }
  @@ -372,7 +372,7 @@ export function dictionaryLiteralForFieldArguments(args) {
  return wrap('[', join(args.map(arg => {
  return `"${arg.name}": ${expressionFromValue(arg.value)}`;
  - }), ', '), ']');
  + }), ', ') || ':', ']');
  }
  export function propertiesFromFields(context, fields) {
  