WorkerConfiguration names should not be allowed to be empty

When load_export tries to create a worker configuration, it never sets a name because we do not export configuration names. Django doesn't mind that at all and uses the empty string, but we have a unique constraint on configuration names per worker. This means that importing any export with two worker runs that use two configurations on the same worker will have errors, because importing the second worker run will create a duplicate configuration.

We can fix load_export, but we should most importantly never allow configuration names to be empty so this issue doesn't occur again.