Enable strictNullChecks

[philipjackson]

We have strictNullChecks enabled in other repos but not this one. This has created problems at least once now (see #673 ), and strictNullChecks is considered good practice anyway, so we should do it, but it will create a lot of type errors we'll have to fix (198 when I last checked). Most of these will come down to adding | null to a bunch of types though so hopefully not too difficult.

We might wish to do this by moving tsconfig.json out into a separate repo, like we did with the eslint config.