react-hook-form: Watched inputs with `null` value cause TypeError

Describe the bug Watching an input component with a null default value causes an TypeError in utils/getPath

To Reproduce Steps to reproduce the behavior:

1. Go to ‘https://codesandbox.io/s/react-hook-form-useform-template-28yig?file=/src/index.js’
2. Click on ‘Submit’
3. See error

Codesandbox link (Required) https://codesandbox.io/s/react-hook-form-useform-template-28yig?file=/src/index.js

Expected behavior Expect that you can watch an input with null values

Additional context Bug was introduced in version 6.0.6