in Customer service management, contact_type has a default value, it should not.
Out of box client script 'Hide None for contact_type and priority'
If it fits your business needs, this can be disabled so that the option --None-- is not removed from the drop-down option. The contact_type field should then default to this option when creating a new case.
Please note, this client script is also removing the 'None' option from the Priority field. If you would like the option of 'None' to stay removed from Priority field, then this client script should be left active and line 3 of the script can be commented out. In this case, None will not be an option for Priority, but record will default to None for contact_type.