wp_insert_user allows a user to be created with empty passwords

[ancawonka]

While looking at the different files where user information is created, I noticed that there are some differences between wp_insert_user(programmatic creation of users) and edit_user (called from the admin).

wp_insert_user assumes that a user_pass parameter is included, which creates a user with no password.

Now checking for empty (or undefined) passwords in wp_create_user

properly annoted and put in its own test function

Removed superfluous !isset() check