hook wp_pre_insert_user_data should return a WP_Error when returned empty

[juliobox]

(This is a follow-up to #40545.)

Hello there.

In wp_insert_user() there is a lot of check of valid data or not, but since wp_pre_insert_user_data, the returned value is not checked at all.

I know, you'll say "hey, you have to return something !" well, yes, but I want to shortcut and forbid this, so actually I'm returning __return_empty_array which is doing the job well, BUT is the admin area we still got the message "New user created." like nothing happened.

I think we should add a new control here. (see patch)

[SergeyBiryukov]

In 45858:

Users: In wp_insert_user(), account for the wp_pre_insert_user_data filter returning empty data.

Props juliobox, SergeyBiryukov.
Fixes #47902.

[SergeyBiryukov]

In 45859:

Tests: Remove $wpdb global reference from test_wp_insert_user_with_empty_data().

See #47902.