refactor: consolidate 0-slots handling to rely on markdown

[melissag-ensemble]

Description

Reduce the three ways we end up with 0 slots to a single approach.

Previous approaches:

• HorizontalLine - slots was forced to [], so totalSlots = repeat * 0 = 0
• Fragment / iframe - repeat was forced to 0, so totalSlots = 0 * slots.length = 0
• Missing/empty slots - When slots is missing or empty, parsing yields [], so totalSlots = repeat * 0 = 0

All three produced 0 nodes consumed as slot content, but via different logic. This PR consolidates on # 3 to treat the markdown document as the source of truth - if a block has no slots attribute (or it's empty), treat it as 0 slots.

Test

• HorizontalLine: testing blocked because HorizontalLine isn't rendered on stage: https://stage--adp-devsite-stage--adobedocs.aem.page/github-actions-test/test/test-hr-0
• Fragment: https://stage--adp-devsite--adobedocs.aem.page/github-actions-test/test/fragment

• iframe: https://stage--adp-devsite--adobedocs.aem.page/github-actions-test/test/iframe

• GetCredentials + RedoclyAPIBlock: https://stage--adp-devsite-stage--adobedocs.aem.page/cja-apis/docs/api

• RedoclyAPIBlock: https://stage--adp-devsite-stage--adobedocs.aem.page/github-actions-test/blocks/redoclyapiblock/redocly-api-block-default