Handle ref.scope.getBinding(...) returning undefined.

The types allow for returning `undefined`, but we always assumed it
would return a reference binding.

Note: this does not fix the underlying tests, they now fail to add the
module imports for `createTemplateFactory`:

```
 FAIL  __tests__/tests.js
  ● htmlbars-inline-precompile › does not error when transpiling multiple modules with a single pl
ugin config

    expect(received).toMatchInlineSnapshot(snapshot)

    Snapshot name: `htmlbars-inline-precompile does not error when transpiling multiple modules wi
th a single plugin config 2`

    - Snapshot  - 2
    + Received  + 0

    - import { createTemplateFactory as _createTemplateFactory } from "@ember/template-factory";
    -
      var compiled = _createTemplateFactory(
      /*
        hello
      */
      "precompiled(hello)");

      84 |     );
      85 |
    > 86 |     expect(transpiled).toMatchInlineSnapshot(`
         |                        ^
      87 |       "import { createTemplateFactory as _createTemplateFactory } from \\"@ember/templa
te-factory\\";
      88 |
      89 |       var compiled = _createTemplateFactory(

      at Object.toMatchInlineSnapshot (__tests__/tests.js:86:24)
```

src/util.js

@@ -13,6 +13,8 @@ module.exports.registerRefs = (newPath, getRefPaths) => {
 
   for (let ref of refPaths) {
     let binding = ref.scope.getBinding(ref.node.name);
-    binding.reference(ref);
+    if (binding !== undefined) {
+      binding.reference(ref);
+    }
   }
 };