Ensure consistent spacing for checkout input fields

[nielslange]

Describe the bug

In #5633, we adjusted the spacing of the Create an account? checkbox below the email address input field. While testing the release, I noticed that this fix caused a regression bug. When displaying the Create an account? checkbox, the email address input field gets pushed down. This problem is caused by:

https://github.com/woocommerce/woocommerce-gutenberg-products-block/blob/360827baff0b00d6852882225ecfc8d5b071c75a/assets/js/base/components/text-input/style.scss#L113-L115

Once the Create an account? checkbox is visible, the email address input field is no longer the only child and instead of margin-top: 0 this element is using margin-top: 1.5em.

To reproduce

1. Go to WP Admin » WooCommerce » Settings » Account & Privacy.
2. Activate the option Allow customers to create an account during checkout in the section Account creation.
3. Create a test page and add the checkout block to it.
4. Click on the email address input field.
5. Activate the option Allow shoppers to sign up for a user account during checkout in the section Account options in the sidebar.
6. See that the email address input field gets pushed down as soon as the Create an account? checkbox becomes visible.

Screen recording

Expected behavior

When adding the Create an account? checkbox, the spacing of the email address input field should remain untouched.

[tarunvijwani]

I'd like to work on this.