code: function maybeAddEmptyStringAtEnd(elements, items) {
    let length = elements.length;
    if (!length || elements[length - 1].type !== TEMPLATE_LITERAL_PORTION) {
      items.push(createStringLiteralExpression(null, '""'));
    }
  }
docs: Adds an empty string at the end if needed. This is needed in case the
  template literal does not end with a literal portion.
  @param {Array<ParseTree>} elements
  @param {Array<ParseTree>} items This is the array that gets mutated.