Corrected setUser TS interface to allow for null arguments

[agrzegorzewski]

Goal

The setUser method in the notifier can take null as a parameter to clear the value from the user object. The current TypeScript interface only allows for string | undefined, which causes type errors when trying to pass null as a parameter.

BugSnag documentation mentions passing null to setUser as a possibility.

Design

The type definition has been updated to accurately reflect the capabilities od the notifier.

Changeset

The type definition for setUser's arguments has been extended from id?: string to id?: string | null.

Testing

The change was tested manually.

[mclack]

Highlighted in #1259 (comment)

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	45.89 kB	13.78 kB
After	45.89 kB	13.78 kB
±	No change	No change

code coverage diff

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