feat(ep-commerce): add EPSearchEmpty for zero-result state

[field123]

Part of #304 — ships item 9 from the catalog-search coverage gaps roadmap (Suggested Sequencing #1).

Summary

Designers drop EPSearchEmpty as a sibling of EPSearchHits and it renders heading + body copy when (and only when) a real InstantSearch response lands with zero hits.

• Visibility gate is useInstantSearch().results !== null && results.nbHits === 0 — deliberately stricter than useStats().nbHits. Stays silent during the SSR-to-first-response window (results is null) and during search-time errors (results keeps last successful value). Adapter-init failures remain the Provider's errorContent concern.
• Mock + Inner split per L7: editor branch renders the wrapper unconditionally without ever calling useInstantSearch; runtime branch gates and shares the same JSX wrapper so the headless-styling contract sees structurally equivalent leaves either way.
• role=\"status\" on the wrapper — implicitly aria-live=\"polite\" per ARIA spec — so screen readers announce the slot's content when results flip from N hits to 0.
• data-ep-search-empty is the documented leaf for designer className forwarding and the headless :where() block (width: 100%; align-self: stretch; text-align: center).
• Default slot ships heading + body in a centered vbox (h2: \"No results found\", p: \"Try clearing your filters or searching for something else.\") — recognisable shape on fresh drop, all values overridable in Studio.
• Sibling visibility is designer's call via dataCond: $ctx.searchStatsData.nbHits > 0; Empty stays single-purpose and does not publish a redundant searchIsEmpty flag.

Design decisions resolved (/grill-me walkthrough)

#	Decision
Visibility gate	results !== null && results.nbHits === 0 (no flicker, no error masking)
Errors	Out of scope — Provider's errorContent handles them
Sibling coordination	Designer-wired via existing searchStatsData.nbHits context
DOM ownership	Single wrapper <div data-ep-search-empty role=\"status\">
Default copy	Heading + body, no embedded CTA, no dynamic query echo
Preview state	L7 \"auto\" | \"withData\" with Mock + Inner split
A11y	role=\"status\" baked, no explicit aria-live (implied)
Headless CSS	width: 100%; align-self: stretch; text-align: center
Props API	Minimal — children, className, previewState; parent enforced
Default tree	h2 + p in centered vbox, gray-500 body

Test plan

• 193/193 catalog-search jest tests pass (yarn test plasmicpkgs/commerce-providers/elastic-path/src/catalog-search/__tests__/)
• 16 new EPSearchEmpty cases: meta enforcement, gate (null / 0 / >0), wrapper role + data attrs, mock branch (Inner never mounts in canvas), previewState=\"withData\" override, lifecycle flips (5→0 mounts, 0→5 unmounts cleanly), default-slot shape, registration
• Reviewer: drop EP Search Empty next to EP Search Hits on a search page, verify it renders only on zero-hit responses and shows the default heading + body
• Reviewer: verify role=\"status\" is announced by VoiceOver / NVDA when filters narrow to zero
• Reviewer: confirm fresh drop styling matches storefront retail conventions (centered, readable hierarchy)

What's next from #304

Per the issue's Suggested Sequencing, after this lands the next item is #7 EPToggleRefinement (single-slot wrapper-onClick for boolean filters like "In stock only").