[login-form] Footer reserves space even if not used

[jouni]

Description

The footer part is always rendered, even when not used (no footer content, no i18n.additionalInformation). This adds extra white space after the form content, because of the gap in base styles.

Expected outcome

No extra gap rendered after the last visible form element.

Minimal reproducible example

<vaadin-login-form></vaadin-login-form>

Steps to reproduce

Environment

Browsers

No response

[web-padawan]

We could probably add has-footer attribute set based on the presence of the content to make styling easier.