fix(v2): Allow footer logo at attribute to be empty

[aarongarciah]

Motivation

Enable the footer logo to have an empty alt="" attribute. Basically, the same that #3352 did for the navbar logo but now for the footer.

See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Img#Attributes

Setting this attribute to an empty string (alt="") indicates that this image is not a key part of the content (it’s decoration or a tracking pixel), and that non-visual browsers may omit it from rendering.

Since the logo normally is decorative an empty alt is fine.

Fixes #3816

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Run unit tests.
2. Create a new project and set the footer.logo.alt option in the Docusaurus config file to '' (alt: '')
3. yarn start and check that Docusaurus doesn't complain about footer.logo.alt being empty.

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[slorber]

LGTM thanks :)

don't forget to sign the CLA, will merge when the CI checks pass

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