API allow blank username for media type of email with authentication set to "Username and password".

Steps to reproduce:

• Open media type create or update form.
• Select type to 'Email' when type is different.
• Set "Authentication" as "Username and password".
• Fill only password field.
• Submit form.

Result:
Email media type is created or updated without username.
Expected:
Media type is not created nor updated. Error message is shown.