Minor improvments to NSE code

Summary:
This differential addresses two issues that Ashoat discovered in my past diffs. The first one: https://phab.comm.dev/D7814, the second one:
https://phab.comm.dev/D8795.

Test Plan: Build iOS app and ensure encrypted notifications are working as previously.

Reviewers: tomek, ashoat

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9400