how to add "Null" value as default option for select list custom field in JIRA?

[Rahul Arora]

I want to keep "null" as default option in the select list, Do we have option to do the same through backend or from UI admin?

[Udo Brand]

If you don't define a default for a non mandatory select list you will have "none" which represents your "null" as default.

You can change the default in the  configuration of this custom field (as I said none is only available if this field is not mandatory)