Accessibility: Themes: Use a default empty alt attribute for the non-linked...

Accessibility: Themes: Use a default empty alt attribute for the non-linked custom logo on the home page.

After [48039] it became clear that the non-linked custom logo on the home page needs an empty alt attribute, as in most of the cases the logo is decorative and doesn't need its purpose to be described.

This change outputs an empty alt attribute by default for the custom logo on the home page. If necessary, it is possible to use the new 'get_custom_logo_image_attributes' filter to manipulate the default attributes for the logo image and set an alt attribute.

Props FlorianBrinkmann, Soean, sabernhardt, audrasjb, SergeyBiryukov, samful, knutsp.
See #36640.
Fixes #37011.

Built from https://develop.svn.wordpress.org/trunk@48283


git-svn-id: https://core.svn.wordpress.org/trunk@48052 1a063a9b-81f0-0310-95a4-ce76da25c4cd