[HEAP-49278] Fix capture when React Navigation rootState is null

[TuurDutoit]

Description

Fixes #366

In some cases, react-navigation's getRootState() can return undefined, as documented here and here. This was being passed to getActiveRouteProps(), causing it to throw: Cannot read property 'routes' of undefined

Test Plan

We applied this change as a patch in our app first, and confirmed it removes the error and successfully sends events. I also added some unit tests for this behaviour here.

Checklist

• [x] Detox tests pass
  • They failed to builld for me. I don't expect my changes to affect any of those tests though.
• [x] If this is a bugfix/feature, the changelog has been updated

[bnickel]

I plan to include this in a release this week.