fix a handful of strict null errors; make test config also be strict

These errors only arose because I didn't run `npm build`, only `npm
test`. Then, when I tried building the null checks failed. Keeping the
two config fiels in sync will help prevent this issue in the future

ts/test-tsconfig.json

@@ -1,7 +1,12 @@
 {
     "compilerOptions": {
         "target": "es6",
+        "strictNullChecks": true,
+        "sourceMap": true,
+        "removeComments": true,
+        "preserveConstEnums": true,
         "outDir": "../build-tests",
+        "sourceRoot": "./",
         "rootDir": "./",
         "module": "commonjs"
     }