Return empty change set

api/s0_base.py

@@ -96,7 +96,7 @@ def get_patterns(name: str) -> list[str]:
 
 def add_node(name: str, node_type: str, id: str, x: float, y: float, table_sql: str, table_undo_sql: str) -> ChangeSet:
     if is_node(name, id):
-        return
+        return ChangeSet()
 
     with conn[name].cursor() as cur:
         sql = f"insert into _node (id, type) values ('{id}', '{node_type}'); "
@@ -117,7 +117,7 @@ def add_node(name: str, node_type: str, id: str, x: float, y: float, table_sql:
 
 def delete_node(name: str, node_type: str, id: str, table_sql: str, table_undo_sql: str) -> ChangeSet:
     if not is_node(name, id):
-        return
+        return ChangeSet()
 
     with conn[name].cursor(row_factory=dict_row) as cur:
         cur.execute(f"select * from coordinates where node = '{id}'")
@@ -145,7 +145,7 @@ def delete_node(name: str, node_type: str, id: str, table_sql: str, table_undo_s
 
 def add_link(name: str, link_type: str, id: str, table_sql: str, table_undo_sql: str) -> ChangeSet:
     if is_link(name, id):
-        return
+        return ChangeSet()
 
     with conn[name].cursor() as cur:
         sql = f"insert into _link (id, type) values ('{id}', '{link_type}'); "
@@ -164,7 +164,7 @@ def add_link(name: str, link_type: str, id: str, table_sql: str, table_undo_sql:
 
 def delete_link(name: str, link_type: str, id: str, table_sql: str, table_undo_sql: str) -> ChangeSet:
     if not is_node(name, id):
-        return
+        return ChangeSet()
 
     with conn[name].cursor(row_factory=dict_row) as cur:
         sql = table_sql