Make DB.replace delegate to DB.ensure when there are no update columns.

This is especially common in tables where the only columns are primary
keys.

doc/changes/DM-46631.bugfix.md

@@ -0,0 +1 @@
+Fix inserts with `replace=True` on dimensions with only primary key columns.

python/lsst/daf/butler/registry/databases/postgresql.py

@@ -351,6 +351,9 @@ def replace(self, table: sqlalchemy.schema.Table, *rows: dict) -> None:
             for column in table.columns
             if column.name not in table.primary_key
         }
+        if not data:
+            self.ensure(table, *rows)
+            return
         query = query.on_conflict_do_update(constraint=table.primary_key, set_=data)
         with self._transaction() as (_, connection):
             connection.execute(query, rows)

python/lsst/daf/butler/registry/databases/sqlite.py

@@ -355,6 +355,9 @@ def replace(self, table: sqlalchemy.schema.Table, *rows: dict) -> None:
             for column in table.columns
             if column.name not in table.primary_key
         }
+        if not data:
+            self.ensure(table, *rows)
+            return
         query = query.on_conflict_do_update(index_elements=table.primary_key, set_=data)
         with self._transaction() as (_, connection):
             connection.execute(query, rows)