chore(null): small strict null fixes

[madalynrose]

Details

This PR took all the yarn null:find results that only had a small number of errors per file, added them to tsconfig.strictNullChecks.json, fixed the resulting errors, and cleaned up with yarn null:autoadd after.

yarn:null:progress before: 64% complete (963/1515 non-test files)
yarn:null:progress after: 66% complete (995/1515 non-test files)

Motivation

#2869

Pull request checklist

• Addresses an existing issue: Codebase should use strict null checks #2869
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.