"Add New Site" screen reports invalid email addresses as missing

[kawauso]

The new site user's email is passed through sanitize_email() and the screen dies with 'Missing email address.' if it's empty().

sanitize_email() returns an empty string (subject to filtering) on invalid email addresses however, so the following is_email() check for invalid email addresses is never even reached.

Steps to reproduce:

1. Add a new site with an email address such as foo@bar

[kawauso]

No is_email(), move sanitize_email() later

[kawauso]

Patch moves sanitize_email() call after if checking email is empty. Also removes is_email() call since it performs much the same checks.

[jeremyfelt]

Confirmed behavior. This is a pretty strange error message to toss up when an invalid email is entered.

17890.2.diff​ continues to use is_email(), as there is just enough difference in functionality and we should continue to respect the filters that may be extending that. The sanitize_email() use is moved down a bit after we check for a completely missing email address.

[jeremyfelt]

17890.3.diff updates the previous patch to use isset() and a string comparison rather than empty.

Now:

• foo@bar = invalid email address
• = missing email address
• 0 = invalid email address
• foo@bar.org = ok

[jeremyfelt]

In 29877:

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