fix(fetchAlgoliaResults): safely access searchClient credentials

[Haroenv]

It's possible to have a broken / badly copied search client without transporter, and that would throw from 1.9.2 onwards.

This PR makes that access safer and throws a clear error.

CR-3338

[codesandbox-ci]

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Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-instantsearch	Configuration
@algolia/autocomplete-example-playground	Configuration
@algolia/autocomplete-example-preview-panel-in-modal	Configuration
@algolia/autocomplete-example-react-renderer	Configuration
@algolia/autocomplete-example-starter-algolia	Configuration
@algolia/autocomplete-example-starter	Configuration
@algolia/autocomplete-example-reshape	Configuration
@algolia/autocomplete-example-vue	Configuration