Fix useInput default value overrides null

[djhi]

Problem

When a record has a field set to null, useInput overrides it with its default value.

Solution

Don't override null

How To Test

• https://react-admin-storybook-8z1fjtwi0-marmelab.vercel.app/?path=/story/ra-core-form-useinput--default-value
• Use the controls that define the initial record value

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.