Fix error when `strictNullChecks: true`
Fix error when `strictNullChecks: true`

With TypeScript's `strictNullChecks` set to `true`, I receive the following errors:

```
Property 'inactivating' of type 'PathNode[] | undefined' is not assignable to string index type 'PathNode[]'.ts(2411)
Property 'reactivating' of type 'PathNode[] | undefined' is not assignable to string index type 'PathNode[]'.ts(2411)
```

The following change fixes the error.
ui-router/core,ui-router/core