Make Bulk Import entity 'destination_namespace' parameter optional

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Problem statement

BulkImports::Entity model has a required destination_namespace attribute https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/bulk_imports/entity.rb#L41-41

When entity is a Group, sometimes we want to create it for a root namespace, where destination_namespace is nil. If you try to create such entity - an exception is going to be raised.

Update BulkImports::Entity model to allow destination_namespace to be nil when it is a group entity. I think for projects it's still required, as we can't have a project without a namespace associated with it (right?)