[TASK] Switch from NULL field name in default ContentConfigurationProvider

There are already measures in place to view this field name as NULL in the proper places when running on 4.x which is the only time a NULL field value applies. The Flux core _should_ be better suited with having an actual field name for this particular Provider.

Note: You'll want to test this one on a site that uses EXT:fluidcontent to be sure it executes all BE hooks properly still; this could have an effect on the prioritising of Providers.