feat(core): don't schedule search without widgets

[francoischalifour]

Note
This is in the PR series that brings support for React 18 in React InstantSearch Hooks.

Description

This updates the behavior of the start() method to only schedule a search when there are widgets statically added beforehand. Scheduling a search without widget is a unnecessary operation because there are no listeners at this point to use the search results.

This behavior is required for React InstantSearch Hooks with React 18 to support new concurrent features. We don't have any certainty about the order of execution of our effects anymore in <InstantSearch> and useConnector(), so we would sometimes end up triggering a single network request, or sometimes two (from start(), and then from addWidgets()).

This eliminates the problem altogether because our framework-based flavors never rely on the search coming from start(), but on the search coming from addWidgets().

Impact

• This change shouldn't break any current implementations.
• This is also the opportunity to simplify our Vue integration to not rely on the next tick to start the instance.

Related

• feat(hooks): migrate to useSyncExternalStore() react-instantsearch#3489

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