Display better value for nulls in Django admin interface

[zqian]

Describe your problem or feature you'd like added

This is a general setting for Django application:
By default, Django admin UI displays null value as '-' , the default setting of admin.site.empty_value_display.

We may want to set the value to empty space, or others, instead of the default '-' per field. For instance some text like "Never updated" or "No term".

Describe the solution you'd like

This is discovered in #1410, where the fix is to remove data_last_updated value for courses.