Afform - Fix ability to add items to an empty fieldset

[colemanw]

Overview

This fixes a bug in the AfformAdmin GUI which prevents adding items to an empty fieldset.

Before

1. Add an empty fieldset to an afform (or delete all the contents of a fieldset).
2. Save the form and exit the edit screen.
3. Go back to edit the form.
4. Try dragging anything into to that fieldset. It doesn't work.

After

Fixed, with test.

[civibot]

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[seamuslee001]

Looks good to me