chore(null): null strictness fixes for 9/15's top 10 null:find results

[dbjorge]

Description of changes

Another round of the top ten yarn null:find results, plus the extra dependent files that yarn null:autoadd revealed were fixed as a side effect of the first fixes.

The most interesting/notable change here is in the error handling improvements to target-tab-finder.ts; previously, a few edge cases the browser could return to us would result in a string error or a "cannot reference property url of undefined" error, where now they will throw more meaningful Error objects. Tests were updated to accomodate.

Before:

> yarn null:progress

**43%** complete (**573**/1343 non-test files)

After:

> yarn null:progress

**45%** complete (**599**/1343 non-test files)

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.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS