chore(null): strict null checks for assessment-provider and deps

[dbjorge]

Details

This PR adds assessment-builder, assessment-provider, and most individual assessment test-steps to the strict null check config.

The only particularly interesting conversion was in assessments-requirements-filter - I ended up converting a pattern that used a mutating filter callback followed by a map to use a single reduce call to implement a filterMap pattern, so that typescript could see the relevant type information within the same scope instead of being split into several scopes.

## Web strict-null progress

**81%** complete (**1272**/1562 non-test files)

*Contribute at [#2869](https://github.com/microsoft/accessibility-insights-web/issues/2869). Last update: Fri Dec 16 2022*

Motivation

#2869

Context

n/a

Pull request checklist

• Addresses an existing issue: Codebase should use strict null checks #2869
• Ran yarn null:autoadd
• 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.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS