Enable eslint null check rule to work both in the build and VSCode (OfficeDev#1353)

* tsconfig must be in a different place to satisfy eslint

* Tsconfig has to be in two places: once for VScode, one for build

Author: AE-MS

tsconfig.strictNullChecks.json

@@ -0,0 +1,5 @@
+{
+  "compilerOptions": {
+    "strictNullChecks": true
+  }
+}