[MWPW-205908] Add empty states and fix count visibility for speakers/…

[rea16068]

Adds empty states and fixes count visibility for the speakers and featured-products session-page blocks, per MWPW-205908.

Changes

1. Speakers empty state: renders a "Speakers" title + "No speakers available for this session" message instead of leaving a blank card.
2. Featured products empty state: removes the block entirely (el.remove()) when there are no products, matching the pattern used by in-person-banner, chrono-box, event-carousel, and others.
3. Count visibility: both blocks now only show the (N) count in the title once the item count exceeds the visible limit (5 for speakers, 6 for products) — previously the count always rendered even when there was nothing to expand.

Resolves: MWPW-205908

Open question for reviewer

Couldn't locate the :empty CSS fallback in libs-styles.css mentioned as precedent for handling the grid row-gap when the featured-products block is removed from an otherwise-empty .grid-column. Closest related rule is .section.grid (row-gap definition, no :empty variant nearby). Flagging for visual verification since I can't preview from a fork branch (eventlibs override only resolves adobecom-owned branches) — please check if an empty grid column leaves a visible gap and whether a fallback is actually needed.

Test plan

• Unit tests: 18/18 passed
• JS lint: clean
• CSS lint: clean (npx stylelint 'event-libs/v1/c2/blocks/event-*/**/*.css')
• Visual preview not possible from a fork branch — requesting reviewer verify visually, especially the grid-gap question above.

[rea16068]

Closing in favor of #306 — opened from a direct branch on adobecom/event-libs now that I have write access, which also allows proper Milestone, Labels, and visual preview via the eventlibs param.