Template deletion not possible if at least one dataset uses it

[stevenferey]

It is not possible to delete a template when it is used by one or more datasets.

The delete button is present and clickable:

After a deletion request, nothing is deleted from the interface, but an error appears in the server logs:

Local Exception Stack: 
Exception [EclipseLink-4002] (Eclipse Persistence Services - 4.0.1.payara-p1.v202304041433): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: org.postgresql.util.PSQLException: ERROR: update or delete on table "template" violates foreign key constraint "fx_dataset_template_id" on table "dataset"
  Detail: Key (id)=(222) is still referenced from table "dataset".
Error Code: 0
Call: DELETE FROM TEMPLATE WHERE (ID = ?)
	bind => [1 parameter bound]
Query: DeleteObjectQuery(edu.harvard.iq.dataverse.Template@de)

Proposed solution:

when deleting a template, set the template_id of the dataset and datasetfield to null.
The SQL constraint will be respected and the deletion can be performed.