feat(instantsearch): make root indexName optional

[Haroenv]

requires algolia/algoliasearch-helper-js#938

Essentially this allows us to create neat non-inheriting indices like this with only three queries sent:

const App = (
  <InstantSearch>
    <Index indexName="1" indexId="1a" />
    <Index indexName="1" indexId="1b" />
    <Index indexName="2" />
  </InstantSearch>
);

Combined with algolia/react-instantsearch#3690 again, you could also create

// 🚨 this isn't possible yet
const App = (
  <InstantSearch>
    <Index indexName="1">
      {/* other widgets, causing no results */}
      <NoResults>
        <Index indexName="1" parentIndexId={null}>
          {/* display stuff from a separate query,
        without interference of inheritance */}
        </Index>
      </NoResults>
    </Index>
  </InstantSearch>
);

Summary

Allow for more flexible use cases to avoid inheritance of indices, like #5585 (comment), without custom code skipping the root index.

This does mean that if you forget the indexName, there's a new warning, and no rendering.

Result

• IndexName of InstantSearch is optional
• No query gets done for the root index if indexName isn't given

[codesandbox-ci]

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Latest deployment of this branch, based on commit 9653948:

Sandbox	Source
InstantSearch.js	Configuration
react-instantsearch-app	Configuration
example-react-instantsearch-hooks-default-theme	Configuration
example-vue-instantsearch-default-theme	Configuration

[Haroenv]

this and the other references will need to be changed once that version is released