Enable 'strictNullChecks' ts compiler option. (#676)

* Enable 'structNullChecks' ts compiler option.

In the majority of cases, I simply turned stuff like this:

```
function fname(param: T) {
  if (!param) throw Error("invalid param; must not be nullish");
  ...
}
```

into this:

```
function fname(param: T|null|undefined) {
  if (!param) throw Error("invalid param; must not be nullish");
  ...
}
```

A more sensible approach would be eliminate the possibility of
null/undef and eliminate the checks too, but that's a bit more involved
and this patch is already enormous. But I think these sorts of
improvements could be taken advantage of opportunistically as the code
is worked on in the future.

* Add @firebase/auth-types to dev dependencies

And also update @firebase/auth to most recent to match.

Author: rsgowman

package-lock.json

@@ -67,6 +67,7 @@
   "devDependencies": {
     "@firebase/app": "^0.4.23",
     "@firebase/auth": "^0.12.4",
+    "@firebase/auth-types": "^0.8.2",
     "@types/bcrypt": "^2.0.0",
     "@types/chai": "^3.4.34",
     "@types/chai-as-promised": "0.0.29",