Differentiate between invalid and missing admin emails when adding a new site

Check the emptiness of the admin email before using `sanitize_email()` and `is_email()` to determine if the address is valid.

Fixes #17890

Built from https://develop.svn.wordpress.org/trunk@29877


git-svn-id: http://core.svn.wordpress.org/trunk@29634 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Quicktags: remove `accesskey` attributes the buttons and don't add empty `title` attributes, fixes #29358
Built from https://develop.svn.wordpress.org/trunk@29878


git-svn-id: http://core.svn.wordpress.org/trunk@29635 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix support for the `hide` option in wpColorPicker, props tollmanz, fixes #29632
Built from https://develop.svn.wordpress.org/trunk@29879


git-svn-id: http://core.svn.wordpress.org/trunk@29636 1a063a9b-81f0-0310-95a4-ce76da25c4cd