TypeScript considers null and undefined as fine for all variable by
default. This patch enables `strictNullChecks`, which should cause
errors to be thrown as soon as a variable is null or undefined without
having it explicitly decleared for itself.[1]
[1]: https://www.typescriptlang.org/docs/handbook/migrating-from-javascript.html#strict-null--undefined-checks
Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>