Don't throw error if auth isn't provided in client constructor

[mattt]

Alternative to #118
Related to #76
Reverts #77

This PR updates the client constructor to not throw an error if the auth option isn't provided.

Unlike #118, this PR removes the check rather than deferring it to the time of request. I think the API should be responsible for communicating missing credentials with a 401. With #103, we should be set up to surfacing that error correctly.

[zeke]

$ npx np minor

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 replicate 0.16.0 published 🎉