Regression - Missing translation for key: "ra.boolean.null" in NullableBooleanInput

[alrik]

What you were expecting:
The first option of nullable boolean inputs is translated properly.

What happened instead:
The first option of nullable boolean inputs show translation key "ra.boolean.null"

Related code:
Checkout: https://codesandbox.io/s/simple-cypzy
You can reproduce the issue when creating or editing a post. Have a look at the "testField" input.

Other information:

Environment

• React-admin version: 3.0.0-alpha.3
• Last version that did not exhibit the issue (if applicable): 3.0.0-alpha.2
• React version: 16.9.0
• Browser: Chrome 76.0.3809.100

[djhi]

Fixed by #3614