fix(insights): do not throw if insightsClient is undefined in server environments

[dhayab]

Summary
The Autocomplete Insights plugin can be initialized without an insights client, which will dynamically retrieve at runtime. This only works in browser environments, but later parts of the plugin init code wrongly assumed the insightsClient to have been retrieved.

This PR fixes this issue by returning an empty Insights plugin in server environments if insightsClient hasn't been explicitly provided.

Fixes #1218

[codesandbox-ci]

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