Fix template `modified` return value for file templates

[ntsekouras]

Recently we added the date field for template and template parts (https://core.trac.wordpress.org/ticket/65049) and it was noted that the returned value from REST API could return false for file templates. This will be fixed here: https://core.trac.wordpress.org/ticket/65728 for 7.1.

While looking into the above issue, I observed that we have the same issue for modified property since 6.3.

We should update the type to be the same with date to string|null and not return false for file templates.

Trac ticket: https://core.trac.wordpress.org/ticket/65730

Backports: ​https://github.com/WordPress/gutenberg/pull/80733

## What?

Recently we added the date field for template and template parts (https://core.trac.wordpress.org/ticket/65049) and it was noted that the returned value from REST API could return false for file templates. This will be fixed here: https://core.trac.wordpress.org/ticket/65728 for 7.1.

While looking into the above issue, I observed that we have the same issue for modified property since 6.3.

We should update the type to be the same with date to string|null and not return false for file templates.

This PR updates the modified prop to also return null for file templates.

## Testing instructions

1. Log in and ensure you have a block theme enabled
2. Test the templates REST endpoint in dev tools: `const templates = await wp.apiFetch( { path: '/wp/v2/templates?per_page=100' } );. Then inspect templates`
3. Observe that modified is null for file templates.

## Use of AI Tools

Opus 5 with direction, changes and review

Changes will be included here: ​https://github.com/WordPress/wordpress-develop/pull/12709

[ntsekouras]

Changes will be included here: ​https://github.com/WordPress/wordpress-develop/pull/12709