Fix types for setUserId

[elliotdickison]

The types for setUserId in the analytics service module do not allow for passing null, which is the recommended way for clearing out the user id: https://firebase.google.com/docs/reference/android/com/google/firebase/analytics/FirebaseAnalytics#public-void-setuserid-string-id

I updated the types and related documentation to allow null to be passed and added a test. The underlying setUserId function in the analytics functions module already allows for null, so this shouldn't result in any behavioral changes.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: df892706357831b087c986ba782d8d23e12c930a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@firebase/analytics	Patch
@firebase/analytics-compat	Patch
@firebase/analytics-types	Patch
firebase	Patch

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[elliotdickison]

I believe this change is still valid...