Consider using `strict: true` compiler option

When making changes across the codebase, having all the strictness settings turned off means that many kinds of issues that might arise from any given change can *only* be identified by sufficiently-robust test coverage, even when the compiler could readily catch it via a stricter setting. For example, dotansimha/graphql-code-generator#4682 exposes a number of problems which could easily be identified statically and therefore made as a change across the library robustly… but instead does not appear until the test suite runs.

**Describe the solution you'd like**

Progressively enable strictness settings, *especially* `strictNullChecks`. This will almost certainly catch a number of lurking bugs, but will also take some time.

**Describe alternatives you've considered**

You've gotten this far with all the strictness settings turned off, relying on the test suite to catch these things, and could continue to do so.

**Additional context**
N/A