Initialize FormFieldContext with null for improved type safety

[davidinoa]

Purpose

This PR refactors the FormFieldContext to use null as its initial value, enhancing type safety and making the component's behavior more predictable.

Origin of the Change

The need for this refactor was discovered when TypeScript and ESLint raised a warning about a potentially unnecessary check in the useFormField hook:

// This check was flagged as potentially unnecessary
if (!fieldContext) {
  throw new Error('useFormField should be used within <FormField>')
}

Changes

• Modified FormFieldContext initialization to use null instead of an empty object cast to FormFieldContextValue.
• Updated useFormField hook to handle the potentially null context value.
• Adjusted TypeScript types to reflect the new context structure.

Potential Impacts

• This change shouldn't affect existing functionality if the FormField component is used as intended.
• Improves developer experience by catching potential misuse of the useFormField hook earlier.

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