[Typings] setUser optional params
[Typings] setUser optional params

The params to `setUser` should be optional to allow submitting `null` values as coded here:
https://github.com/bugsnag/bugsnag-react-native/blob/master/android/src/main/java/com/bugsnag/BugsnagReactNative.java#L152
https://github.com/bugsnag/bugsnag-react-native/blob/master/cocoa/BugsnagReactNative.m#L192
bugsnag/bugsnag-react-native,bugsnag/bugsnag-react-native,bugsnag/bugsnag-react-native,bugsnag/bugsnag-react-native,bugsnag/bugsnag-react-native