Set rich_text_list with elements attribute of empty array by default

[wilsonehusin]

This changeset initializes RichText::List with elements attribute having [] (empty array).

For rich_text_list, Slack considers elements attribute to be required. As of today, Slack would error out when elements is absent, or when elements is null. However, they accept empty array just fine.

While there is not much point of creating rich_text_list block without any elements, we can avoid invalid_blocks error and still send something to Slack. This is consistent with other block definitions in the gems where required fields has default empty value