Fix required field on complex metadata in a toggleable form

When a field of a complex metadata is required in a toggleable form, it will save an empty value for the full field if it one tries to saved an empty value once.

Steps to reproduce:
Studio

• Create a multivalued complex field with at least one subfield
• Use it in a Form Layout, make the subfield required
• Reference that Form layout as a toggleable form
• Deploy
  Server
• On the document edit the form, edit the form,
• add 2 entries but enter a value for only one of them
• save -> validation Error OK
• Re-hit save : it saves without any value.

(you would have the same behavior with existing values)