[native] fix assumption that in ThreadScreenPruner app route is defined

Summary:
Problem described in ENG-2427.
Probably we want to investigate deeper, maybe there is no need for invariant in other usages of getStateFromNavigatorRoute - if yes I can create an issue.

Test Plan: Invariant should not crash app in dev mode while ThreadScreenPruner is rendered but NavState wasn't fully propagated, e.g. because App Navigator isn't the top of the stack

Reviewers: tomek, inka, ashoat

Reviewed By: tomek, inka, ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5845