fix(metadata): ensure safe user agent detection

[sarahdayan]

Summary

This ensures user agent detection doesn't break in non-browser environments.

In React Native, window.navigator is polyfilled with a custom product property, and doesn't have other properties traditionally exposed in the browser.

This change ensures this code path doesn't yield a runtime error regardless of the environment.

fixes #992

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@algolia/autocomplete-example-playground	Configuration
algolia/autocomplete	Configuration
algolia/autocomplete	Configuration
@algolia/autocomplete-example-starter-algolia	Configuration
@algolia/autocomplete-example-starter	Configuration
algolia/autocomplete	Configuration
@algolia/autocomplete-example-vue	Configuration

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 18e7694:

Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-instantsearch	Configuration
@algolia/autocomplete-example-playground	Configuration
algolia/autocomplete	Configuration
algolia/autocomplete	Configuration
@algolia/autocomplete-example-starter-algolia	Configuration
@algolia/autocomplete-example-starter	Configuration
algolia/autocomplete	Configuration
@algolia/autocomplete-example-vue	Configuration