use strict null checks for added type safety

From the TypeScript manual "In strict null checking mode, the null and
undefined values are not in the domain of every type and are only
assignable to themselves and any (the one exception being that
undefined is also assignable to void)."

In other words, this setting means the compiler will complain if you
try to use a variable, but it might be null or undefined. Since the
project builds and tests pass with it, I think it's good to turn it
on. If it becomes annoying, you can always turn it off later

ts/tsconfig.json

@@ -1,6 +1,7 @@
 {
     "compilerOptions": {
         "target": "es6",
+        "strictNullChecks": true,
         "sourceMap": true,
         "removeComments": true,
         "preserveConstEnums": true,