[native] Don't refetch nonce from identity after identity auth

Summary:
Noticed that generateNonce was getting called after the user auth'd with identity, probably because the effect was re-run.

Adding this condition prevented it from getting rerun unnecessarily.

Test Plan: Confirm that I'm not seeing IDENTITY_GENERATE_NONCE_STARTED and IDENTITY_GENERATE_NONCE_SUCCESS dispatched after identity auth during login

Reviewers: varun, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11771