api/thesauris admits empty groups

[mfacar]

Describe the bug api/thesauris admits empty groups, these values are not valid options for an entity's property.

Expected behavior The endpoint should return a validation error when there are empty groups.

Additional context #7132 contains a validation at the UI Client level when editing through the side panel editor When #6906 gets solved, the UI must implement a validation at the UI Client Level when a group is emptied through dnd.